35%, and definitely do ceramic. You get the most infrared and UV blocking from ceramic tints. That's what affects temperature and interior fade, plus it keeps the bad rays off your skin!
I've only had a few tinted cars (legal limit is 50% in MN, hardly worth the time), and I've found that anything less than 35% light transmission SEVERELY hinders night time driving. I had a car with 5% (limo) tint, and it was downright impossible to drive after dark.
Also, I noticed you mentioned taking a road trip. Another benefit of lighter tint: LESS ATTENTION! You can easily go from a state to state and attract tons of attention just because of tint. It's one of the easiest things to be stopped for, next to a burned out light or expired registration. For example:
Minnesota (my home state) states for a sedan type vehicle:
Windshield: no tint allowed
Front windows: 50% light transmission
Rear windows: 50% " "
Reflectivity: 20% max
South Dakota:
Windshield: Non-reflective allowed over factory tint strip
Front windows: 35% light trans.
Rear windows: 20% " "
Wisconsin:
Windshield: Same as SD
Front windows: 50% light trans.
Rear windows: 35% " "
As you can see, I can drive between 30 and 100 miles in either direction, and have completely different rules to abide by. Now, obviously if your car is registered in a certain state and you have a license from said state, you'd be fine. AFTER the traffic stop. Why give the police a reason to stop you? That's my argument for 35% max on a sedan.
If we're talking an SUV then the laws are mostly in agreement: any tint level behind the driver is fine. Why that is, I have no idea. Lawmakers, I suppose.