RE: t maxx problems pls Help!!!
The OS is probably the better engine. But that's based on owning several OS engine and not any experience with the .21TM. However, the .21TM is around twice the cost of the 3.3 and I'm not convinced that it's twice the motor of the 3.3.
So, between those two, if cost is a consideration go for the 3.3. If you're not worried about spending $$$ then go for the OS.
My next motor won't be either of those though, I think my next one will either be the RB 523 or the Pico .26.