I have 15x6 styled wheels with Cooper Cobra's in 215/65R-15 and highly recommend them and that size.As we all know, good 15" tires are getting hard to come by. I'm pretty set on the RWL look. I can pick up a new set of Cooper Cobra CS5s in 215/60/15, which is not quite my size but the closest I'm likely to get in a tire that is reported to be pretty good. The other option that I am considering, mostly out of frustration, is the Legendary Wheels Styled Steel 17x7" wheels (link here) with 225/50/17 tires.
If you want your Mustang to ride like a nightmare on rough roads go with the 17"