First find out what the legal limit is for tint in your state. The installation shops won't tell you the truth and routinely over tint. If you are stopped and issued a ticket, telling them the shop installed it won't get you out of the ticket.
My thoughts would be go as dark as legal, and look for premium film with a high heat rejection factor. I believe some of the higher-end films offer a metallized layer to block more heat for the same light transmittance. Also, don't forget that metallized film will blind or seriously impair radar/laser detector...if you use that sort of thing.
I have a guy I use who comes out and does the tinting if you really wanted a recommendation. I think his business's name is Bell Window Tint. I don't think who does it is such a big deal so long as they have a decent amount of experience. Mine already had tinting on it when I got it so I haven't had to deal with it, but I did have limo tint (yes illegal) put on the lower rear window in the back. With the surrounding black it makes it pretty much dissapear from the outside which creates a neat little effect.