If you want it to look dark, better go with 20%. I don't like real dark tint, I went with 35%. My interior is light gray, and you can check out my pics by clicking the link in my sig. That may help you decide on what percent tint you want.

As far as cops go, it really varies with where you live. Some states are lenient, others are stingy in what they allow (and enforce).

One thing is for sure...any tint really helps keep things cool in the summer and saves on sun exposure on the interior.

EDIT: Those pics don't show the tint that great. Here's 35% in overcast conditions: