I got my fronts tinted yesterday with 20% and its pretty close to matching the factory rears. I think 15% would be just a tad darker, but it may even out due to the amount of light coming in from the windshield. Ill post up pics as soon I get some shots.
Protg: did you end up getting your windows tinted?
@ghost_ix... Sorry for the late reply, kind of forgot about this thread. I did get my windows tinted over the course of two months or so. Just wanted to chime in with my findings in case anyone is currently at the same situation I was in, decisive on tinting, but unsure what tint % to go with.
Straight to the point, my final results are 15% on front driver and passenger side windows. Get the 15%, you won't regret it and you will save yourself time and grief of having it redone.
My rear factory tints measured out at 23-25%. (not sure how the other user measured at 16%) My first go at it was 25%.... a month later, I went back, removed it, and put on 15%.
Without a doubt, 15% is the perfect match for me from the outside. I don't think you could go wrong with 20% up front either. Anything higher than 20%, you would notice a difference.
From the inside, you can tell a very slight difference in shade between front and rear window. And I mean a very slight difference. When the 25% was on, it was nearly spot on from inside asides the color of the tint (3M premium has a slight greenish hue, whereas factory is black toned). From outside, the difference was notable between front and rear window at all times of day. Even at night time, if you happened to be parked close to or under a street light, you would see the difference. And the closer you are to the car, the more noticeable the difference.
So there you have it, my personal experience and learning curve.
For the ones who put 30-35% up front, I would imagine a big difference during the day. But ultimately, it's what you're comfortable with.