I live in NJ (front windows being tinted at all is illegal, rear windows can be tinted at whatever percentage). On my 03 Black Coupe, I had 15% tint all around and I thought it looked great. IMO, sticking with one tint level all around makes for a cleaner look. Go with 35% if you want the heat/sunlight reduction benefits of tinted windows, you want the tinted look, but you also want a classy and semi-see-through appearance. Go with 15% if you don't want people to see inside your car from more than 10 or 20 feet away even in the sun, and you want it to be pretty freaking dark at night. Backing up/parking at night is difficult with 15%, especially if there is no street lighting around you...I almost always rolled down my windows when backing up at night. I am going to go with 15% tints all around again, with my 06.

Here is a pic of my black 03 with 15%