I wouldn't drop stock jg internals into a vfc or g&p. I'm not saying go buy $80 gears or anything like that, but I prefer shs to JG and they cost only slightly more if not the same . JG neo motors are also slow as poop unless you are using a high voltage battery (not many people are willing to use 11.1/14.8 lipos). Not only that they're a pain to find in stock.
KWA also is a gun which is geared toward people who don't upgrade. They shoot at field limits out of the box and the cqr shoots at cqb limits out of the box and is geared towards being budgeted.
^That being said they can be upgraded. I don't necessarily prefer the AIM brand but the gears do fit the 2gx gearbox along with the full rack piston. They'll put out at least 450 reliably if not more.
The RS svd is good... If you have the $590 + money for a posp style scope.
JG full gearbox is for people who do not want to upgrade(one package with most parts being useful/compatible). I suggest it if someone really have no idea what works; JG works.
JG neo magnet motor is easy to find on ASM for about $20.
They are great because they are actual high torque motors(20+tpa) instead of wannabe-magnums(~16tpa). I run with 32tpa Dboys armature in neo magnet can, by the way. All you need is lower ratio gears. Point of going with ubertorque is obvious: I do not want motor to become hand-warmer any time soon. *cough*SHS*cough*
Also, you better be running giant 11.1v lipo brick if using some sort of Ar15/M14-based airsoft DMR; for trigger response as well as capacity.
KWA is not for people who do not want to upgrade; they have horrible self-shimming and wrong AOE last time I had "pleasure" to work on a broken KWA(bevel gear got eaten up by Matrix "magnum" motor).
KWA is for people who wants to desperately
believe they do not need upgrade(in reality, they obviously do).
AIM is one of worst brand of "upgrade" parts ever? I have hard time deciding which brand of piston is worse: kwa or aim(metal rack with poor support = cave in and die).
Actually, RealSword SVD without upgrade is pretty bad internally; trigger response is slow even with good 11.1v stick(only thing that fits) and accuracy is not all that awesome. If it was so good then I would not be upgrading. But externals are well worth it; it can also mount bayonet for some long-range stabbing. Mostly, I wish it had lower ratio gears but obviously that would not work so well with stock motor.