I'd also say if there's nothign wrong with your motor, there's no need to upgrade. However, I'm a big fan of the Guarder Infinite Torque. I have that in 2 of my guns (including my G36C), and it's a great motor.
However, just to let you know - the new Guarder motor I got (from airsoftparts) had a defect. The pinion gear on Guarder motors is press fit, and the hole in the gear was too wide, causing it to spin freely on the shaft. However, Jugglez, being the exceptional dealer he is, sent me a replacement gear (at no charge) which worked fine.
I have no experience with the other motors, so I don't have an opinion on them one way or the other.
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