205/50/15 is what I'm probably going to go with. I've got 2 BBS Miata M Edition 15s if I could find 2 more I'd put those on.Since no one else is yet, I will argue in favour of the 15" wheels . Lighter, more tire sidewall, and good tire sizes: you can't go wrong between 205/50 and 195/55. Both are very close to stock diameter, whereas 215/40r17 is pretty tall. You'll feel it under acceleration. That's a tire size I would personally save for H-swapped cars. Actually, when I bought my car, I think that's what was on it. I sold those wheels ASAP.
16's are a good balance between the looks of 17's and the performance advantages of 15's. Currently running 16's myself.
The bigger the wheels, the lower the car needs to be in order for it to look good.