As far as lighting goes I think they are a wash in terms of what you need on a pistol.
Advantage TLR series: easier to put in and off the rail. The x300 is quite tight (which, I guess is an advantage if you never wanna take it off)
Advantage x300: much better momentary switch control. I like being able to just push the switch forward
The new version of the X300 is basically a copy of the Streamlight with how it attaches. I personally prefer the old style because there is no worry about the screw loosening and coming undone, however, it is a pain in the ass to put the old style ones on.
Because of the older PITA mounting system of the X300, I really like my X300B. It's a lot easier to pop on and off.