Most of my cars have the 3M, but I have heard good things about Xpel. I don't know the thickness.
My '08 Infiniti SUV got its 3M in July, 2007. I would say it has not yellowed, but perhaps it isn't as clear as it was when new. But otherwise looks great. Our '09 5-series got its 3M in early 2010, and not as clear as when new, but also not yellow.
Wednesday a new-to-me 2012 335is goes in for its clear bra. Whatever the vendor has is what I am going with. He did one of my other cars but I don't recall the brand used. This 335 only has 17k miles on it and looks showroom new.
I agree to let the pros apply it. I have watched videos and I realize I don't have enough patience nor know the tricks to do it myself. I expect to pay around $600 or so, but that is way cheaper than getting the front end painted every few years. Plus I can scrub hard on bugs without damaging paint.