I know this is kind of late but in my opinion i think GT6 is the more rewarding version to play and now with 12.00 being finally feature complete. The tech tree is also much more complex and the crucible mechanics are so rewarding once you figure out how to get your first ingots or get more higher Tier machines. But its kind of hard for newcomers thats when i would recommend GT5u.
In the case of useful mechanics i think GT6 also is better (love me some covers).
Edit: Just to be clear i enjoy both and each have their individual strengths! 
TL;DR: Try GT5u if you are new. Otherweise give GT6 a try but beware that there aren’t many Tutorials!