19s if ur going to lower...20s if ur not

 

What difference will there be if I stick to 35 profile in the rear and 30 in the front besides the extra cushioning?

 

245/35/20 front and 275/30/20 in the rear is the proper staggered set up. As for the ride quality between the 19's and 20's...the difference is almost non existant and should not affect your ride quality upgrading from 18's. The number one concern when upgrading to 20's is potholes...but realistically, how many times have you hit one of those? I've been riding on 20's for a long time, and I have yet to hit one to trash my wheel. If you hit one frequently, I'd suggest you stay with 18's and check your alignment and suspension regularly.

 

http://www.miata.net/garage/tirecalc.html