...Money after all, does control everything.

So for ease of discussion, I see 4 categories (alphabetically listed):
1) Alcon / STaSIS
2) Brembo Aftermarket (includes AP kits)
3) Brembo OEM (via Porsche via ECS Tuning)
4) StopTech

To continue the ease of discussion, I see 3 areas:
1) Calipers
2) Rotor Assemblies
3) Pad Options


So let's dive right in...

Stage 1) Calipers
The Caliper is important along 3 main areas:
1) Number of Pistons
2) Piston Sizing
3) Physical design and size

Stage 1.1) Caliper Number of Pistons
The more pistons you have, the less pad taper you'll get via differential sizing and the longer pad you can put in.

So why differentially sized pistons?
Because the leading edge of the pad is hotter then the trailing edge, and this fact is what makes a 2 piston (or single piston floating) caliper cause the pads to taper. By adding another set of pistons, you can make the first set smaller then the second, thus offsetting the taper from the heat difference.

So why a longer pad?
Very simply, the pad contact area has an affect on total brake force. More pad, more force. You can only make a pad so long relative to the pistons. More pistons, longer pad. Longer pad, more brake force on the same size disc.

Stage 1.2) Piston Sizing
Piston sizing determines the displacement of the caliper. This is important as displacement (along with many other things) determines the brake pedal feel. In literal terms, assuming the same pedal box and master cylinder size, the bigger the pistons, the more the displacement. The more the displacement, the further the pedal must travel. Brake pads have a huge affect on this, but that's not for this section.

So in simple terms, on a B5 S4 with the same pads, a 4 piston caliper with a 36/41 piston sizing will have shorter pedal travel then one with 41/44 piston sizing to get to X amount of force on the disc.

Stage 1.3) Physical design
This covers 2 things, the size of the caliper and the physical design of the caliper,

The size is important as on every car, you'll be trying to fit a wheel around the caliper. The smaller and more efficient the caliper size, the easier this job becomes. The larger the caliper, the more difficult it becomes.

The other is how the caliper is made. In our group of 4, they come down as:

1) Alcon = 1 piece cast aluminum
2) Brembo/AP = 2 piece cast aluminum then bolted
3) Brembo OEM = 2 piece cast aluminum or 1 piece cast aluminum (cayenne caliper)
4) StopTech = 2 piece cast aluminum then bolted

Why does this matter?
Well, the caliper is an inherently bad design. It's got this big hole in the center where all the action is happening. So you want your caliper to be strong.

Why 1 piece vs. 2 piece?
This comes down to manufacturing costs. Since all are samples are cast aluminum, it ultimately comes down to machining. A 2 piece calipier means you can easily machine the piston bores with the caliper in halves, then bolt it together. With a 1 piece, you must machine the piston bores from the inside of the caliper, this is more difficult.

Why 1 piece vs 2 piece continued...
None of the kits offer a forged caliper, but this would be the best if it was offered. Just like cast wheels vs forged wheels, forged is stronger and lighter. But, it's more expensive. None of the current kits offered a billet caliper either, they are all cast, but billet is better then cast.

Why 1 piece vs 2 piece continued continued...
So in teh world of making things out of aluminum:
1) Forged (best)
2) Billet (better)
3) Cast (ok)


Stage 2) Rotor Assemblies
We'll assume they are all floating 2 piece designs, what makes them unique?

1) Floating Arrangement
2) Veins
3) Manufacturing

Stage 2.1) Floating Arrangement
There are two main types of floating arrangements:
1) Floating Rotors
2) Floating Hats

Floating Hats...
Floating hats are the best, but they have draw backs. First, they wear down the hat, which is costly to replace. Secondly, they are typically harder to make "quiet" (i.e. no rattling). Floating hats have a rotor with a bolt circle comprised of a normal hole. The rotor attaches to a "T" bobbin solidly. The "T" bobbin slides up and down on a hat (aluminum/steel/other) that has slots cut in it.

Floating Rotors...
Floating rotors do not attach solidly to the fixture, but rather the fixture attaches to the hat, and the rotor floats on pins or bobbins. These rotors have an oblong hole cut in the bolt circle. This means the rotor is taking more abuse, as the movement of float is at the rotor, not the hat, so there is more stress on the bolt circle (really on the flanges that have the holes). As a result, floating rotors have larger flanges then floating hat rotors. By larger, they are thicker.

This is easier to "quiet" cause you can add a spring between the fixture pushing against the rotor flange. But, that spring is occupying the space used to allow the disc to grow, so just like any spring, the amount of pressure and the amount of travel is key to keep the rotor truly floating.

Springs on rotors...
Brembo uses "McLaren springs", StopTech, STaSIS, ECS all use belleville washers (conical shaped). I think StopTech springs every fastener, where as Brembo and STaSIS spring every other fastener. You could probably get away with every other other (but you'd need to run a 12 point bolt circle, most run 10 point, and 10 does not divide by 3, so you get every other). All the springs do is push the rotor against the hat, keeping it from making noise.

So floating...
1) Floated Hat (best)
2) Floated Rotor (better)
3) No Float (ok)

Stage 2.2) Veigns
Rotors are basically a pump. The better they move air through them, the better they can get the heat exchanged, and be ready for more abuse. I don't know what style of directional veign is best, but the race car rotor manufacturers seem to be adding more veigns to improve the disc. That being said, we'll just assume that directional is far better then straight veigned and leave it at that.

Stage 2.3) Manufacturing
A rotor is a cast and machined part just like the caliper. The material is designed around taking abuse, so the metal makeup should focus on that end goal. So whatever goes into the casting is going to affect how well it works on your car. But good luck getting any information on this from any brake company. That's like calling Michelin and asking them what they put in their F1 tires.

The other side is the machining aspect. This is critical. Especialy in a floating setup. Bear in mind that each piece of a floating rotor is a machined part. And each piece has a tolerance. The more pieces, the more tolerances. The hat, the bobbins/pins, the springs, the rotor flange, all of these contribute to how much the rotor can float and grow. The better the machining, the more consistent. Each machine shop will specify what tolerance they can operate at. Hint Hint Hint, the better the machine shop, the better the tolerance, the more expensive the part, the better it works time and time again.


Stage 3) Pad Options
Pads have the single largest affect on braking. They are like tires. You can spend thousands searching for the 20%, or you can focus on the 80%. Everything above this is the 20% of what makes good calipers great. Pads are the 80% of what makes the brakes work and "feel" good. One caveat to this is that displacement of the caliper has a huge affect on BIAS, which has a huge affect on brake FEEL. But none of the products are really bad, they are all just different at their bias.

One last thing on bias...
Say the StopTech 332 front kit has the smallest pistons (least displacement) and the Brembo 355 kit has the largest pistons (most displacement), using the same pads front and rear these two setups will have different pedal travel characteristics. BUT...put an aggressive pad on the rear of the Brembo equipped car, and a soft pad on the front of the StopTech equipped car, and you end up with two cars that are very different, that brake kinda along the same lines. Pad have a huge affect, that's the point here.

Pad availability:
Some calipers like the Brembo stuff, or StopTech for that matter since they used a common Brembo pad shape, you'll have tons of options. Alcon you'll have less, but since a ton of teams race on Alcon, you still have a good selection. Some of the more unique OEM stuff (Cayenne caliper) you'll have limited options. Also on OEM, limited options can often mean expensive options (like OEM B5 S4 pads).