My previous engine was a GRP Tuned .21 and I have been more recently running a Ninja JX21, and I can honetsly tell you that the Ninja JX21 is an overall faster engine. It has more top end and a HUGE ammount more acceleration and bottom end in general, that said i wouldn't necessarily say that its the better of the two engines. I have found that both engines will run for 10+ in a buggy, although the Ninja JX21 does seem to be abit more economical which again coupled with the extra performance is pretty amazing.
Despite the fact that the Ninja JX21 is the faster of the two engines, I do have to say that overall the GRP Tuned .21 had a smoother and far more controllable powerband - At least for me, I found that the softness at the bottom end made the car so much easier and predictable to drive, particularly in low grip conditions. Both engines I found to be very reliable though, and a breeze to tune.