From personal experience
15x6: best choice is 205/65 or a 215/60, you can get a 225/60 on and it will look pretty bulgy/awesome but I personally do think the bigger sidewall and bulge does affect handling a fair amount on a 6" wide rim.
15x7: 215/60 or a 225/60, either are a good choice
Really all depends what is important to you.
Handling: 215/60.... due to the smaller sidewall and wont have much bulge due to being on a 7" rim
Straight line: 225/60... more contact with the road, that said, a lot of people wont notice the difference in handling
Sidewall sizes
205/65: 133.25mm
215/60: 129mm
225/60: 135mm
also 205's, look pretty "meh" on a 7" wide rim.. Keep them on 6"
I've used 205, 215 and 225's on a variety of 6 and 7" rims, so this is my personal opinion but it's up to you to make your decision