The 225/60/15's will be slightly less squeezed on the 15x7's. They will fit on a 15x6", but the narrower rim tends to push the center of the tread patch up a little bit because the sidewalls are squeezed closer together.
It won't be a huge difference but you should get slightly better performance out of a 225/60/15 on a 7" rim than you would on a 6". Not saying you'll actually notice that difference because it will be really minor, but that's the idea. If I were buying new rims with that tire size in mind I'd buy 15x7's. If I already had a set of 15x6's I wouldn't worry about it.
