The Trek is steel and has 26' wheels compared to the aluminium Rockrider's 27.5" (depending on the frame size you are after).
Other than those fundamental differences they are very similar, so it would come down to your preference on those two points.
The Trek also has slightly less travel in the fork (and doesn't mention if it's lockable that I can see, whereas it reads like the Rockrider is), but I doubt you'd notice 5mm.
Personally I would go with the Trek.