but I believe the 996 uses a monobloc one piece design... vs the traditional multi piece caliper. I'm sure there are other differences too, such a large change in the design always warrants including a few other changes too.

Pistons are the same but the 996 is a Monoblock design, fronts want a 330x34mm wide rotor, 993tt is 2 piece design, fronts want a 322x32mm wide rotor. Either rear wants a 322x28mm rotor.

996tt are more compact and lighter

<img src="http://forums.pelicanparts.com/uploads5/996ttmono_BR21113617744.jpg">

...so same clamping force. 996tt saves about 1.5 lbs per caliper of 993tt and the Monobloc is physically smaller. 996tt also has slight edge in pad size (154 vs. 151 cm2).

993tt is cheaper and has quick-change pad retainer (better).

Jimmy