I've owned TM Glock 17 and now own a WE G17. Externally, the TM definitely feels a lot better than the WE. WE Feels like it's made of cheap plastic.
That being said, in the performance category, the WE actually is able to hold it's own. The WE is propane ready out of the box, and is actually very consistent in velocity and accuracy. On the other hand, the plastic slide on my TM broke within a month's use on propane, which forced me to spend $70 on a metal slide.
At the end of the day if your budget isn't restricted, then get the TM and a metal slide, or use duster gas. If you're on a tighter budget, get the WE; it can handle propane out of the box, and performs almost as good as a stock TM.
World record in propane sniffing.
-Tokyo Marui M4 MWS GBBR
-ICS CXP M4
-KJW KP07 (1911 MEU)