I personally opt for the professional installer for two primary reasons:
1) Quality of the tint film
2) Good installers do a better and faster job than me
The quality of the film affects clarity, heat protection, glare reduction, and UV rejection. Cheaper films over time may 1) fade (to purple), 2) bubble, 3) peel (usually edges), craze/crack, and delaminate(buckling/orange peel effect).
Good quality film usually utilizes superior materials (polymers, dyes, adhesives) in their manufacturing process. High end tints today are significantly more advanced than the ones around ten years ago. They usually also offer several options (metallic v. non-metallic) and quality levels. The quality of adhesive is often an overlooked factor as inferior adhesives will often degrade faster and result in delamination, bubbling, cracking, etc.
Solargard, 3M, Johnson, Llumar/Formula One, Madico, and Huper Optiks are some of the manufacturers that have good reputations. Although each has a pretty wide range of quality between their product lines.
Qualified installers will also make a difference in the long term effectiveness of the tint. Some tint manufacturers (e.g., Huper Optiks) require their installers to go through specific training. While the training is usually pretty basic, it does ensure that common mistakes are not made. I personally like the fact that good shops will warranty their work.
I had my FJC tinted with Madico 35%. It is a bit lighter than the factory rear tints but I wanted to stay legal. The installers I used charged $75 for the two front windows and it took two guys 25 minutes, and came with a lifetime guarantee.