The tint that comes from the factory is in the glass and not a film. I've had all more cars tinted for the reason of cutting down the UV. I had all the windows tinted on my wife's Q5. You'll find some pics of it here
I used Premier Auto Group
on my last 2 cars and Auto Mall Tint Specialist
on my first 2. The reason I switched was I was having a clear bra installed on my Cayenne and wanted the convenience of having it done all in one place.
Both places do great work, so it really comes down to the film selection. Premier only carries Spectra Photosync, which they claim has superior heat rejection. The Auto Mall Tint guys carry everything from the cheap stuff to really good stuff. I'd recommend Huper Optik if you don't go with Spectra. I had Suntek on my last car and I can tell you the clarity and heat rejection on the Spectra is way better. I had Huper Optik on my first car and the heat rejection was comparable, but it's been so long I can't really compare the clarity.
Be warned that since we live in the Bay Area, everything is excessively expensive. I don't know how much the Auto Mall Tint charges these days. If you want to know how much Premier charged, PM me.
Here are some pics
of the work Premier did for me on my CTTS.