The most important thing to look for is a good reputable installer. The product still makes a difference since some are more optically clear than others, some are more resilient to discolouring from uv exposure and some have top coats like Clearshield.
It's best if you find an installer that can cover entire panels, this will virtually hide the edges of the product and make it pretty much unnoticable. But you have to find a shop that stocks really wide rolls which is expensive and they have to have the skills to install it as well.
Here are a few examples of my work.
SpeedMerchants Motorsports - Paint Protection Film