I went 20% once on a vehicle and regretted it. 20% was way too hard to see through at night when you're trying to back your vehicle up. 37% I had done on a couple of other vehicles and was very happy with the compromise.
20% looks cool in the daytime but it sucks at night if you ever back your vehicle up.
My Escalade was bought used with 20% on the front windows to match the factory tint, I hate it at night and will probably have it stripped off and replaced with 37%. Right now I roll the windows down when backing up at night (really sucks in the rain) so I can see the mirrors better.
I lived in Kalifornia for 15 years where any tint on the front windows was illegal. They didn't enforce it much though (wouldn't pull you over for just tint). They'd let you keep it as a built in excuse to pull you over if they ever wanted to for some other reason. So you might want to think about that as well if you're ever worried about being pulled over and being hassled like on holiday weekends whey they're going harder for DUIs, see you leaving a bar parking lot, etc.
The built in excuse to pull you over is convenient for law enforcement. My friend got pulled one night leaving the local bar (cop parked in lot across the street). She didn't do anything wrong as far as driving, she was given the excuse on pulling her over of her pine tree air freshener hanging from the rear view mirror. Did you know in most places it is illegal to have anything (air freseners, parking passes, etc) hanging from your rear view mirror?
While he had her pulled over, of course it turned into a DUI check.