ie 17x7 vs 17x7.5 vs 17x8, if the exact same wheel was offered in the all three widths.

I'm assuming this is for a B5 A4, so you'll be using something like 225/45/17 tires with these rims.

In this case, I would choose 17x8 or 17x7.5 at the least. 225-tire will not look that great on a 17x7 rim, although it will provide the most curb-rash protection because it will be sticking out very far.

With 17x8, a 225-tire will be pretty much flush with the rim lip and will therefore result in less sidewall flex and hence quicker steering response. 17x8 also gives you the flexibility of using wider tires (235, 245) if you ever wanted it. With 17x7, 225 is the max you can go.

Now, if you're planning on using 205/50/17 tires, then 17x7 or 17x7.5 would be best because this tire would look too stretched out on a 17x8 rim. So, as I said, a lot depends on the tire size that you'll be using.

I'm running 17x8 with 225/45/17 tires.