It all depends on what you are looking for out of window films.
If you are only trying to protect your interior from UV from the sun. I'd go with 50%. They're hardly noticeable. And you won't get pulled over by police.
If you are going for the look, 20% would be nice, but depending on the state laws, you can get pulled over if applied on the front side-windows.
Since your interior is gray, meaning not being light like ivory, 35% wouldn't be too bad. My car is silver w/ gray interior, and I have pretty much 35% all around. Feel free to check out my ride tread.
This is my opinion, but although the darker the better it looks in general, going anything darker than 20% would be difficult to see through the outside in the night. I.e., when changing your lane in a tunnel, or backing up from a lot in the night. (Regarding the poor rear-view issue by dark films through a mirror, you can get away with having a Broadway rear-view mirror that's designed for a tinted windows tho.)
GL w/ your decisions.