I typed up a response on the phone but apparently it didn't show up;)
I haven't handled the JG version only TM since that's what I've got. From what I've heard the only real difference is the quality of the stock, TM being better. Though that said most people replace nearly the entire internals so hard to say there. On my TM build I did it as a budget build, I already owned the gun from when I had more money to spend. So with my build I put in an rhop, upgraded the sears, spring, and probably a few minor things I'm forgetting as well as some DIY mods. That has given me a reliable gun that shoots well at 100 yards fairly accurately and goes further than that but the accuracy suffers. That said I'm not sure how much would be gained by replacing things like the hopup chamber etc. I don't have any curve issues or the like that people typically complain about with the stock hopup chamber. So my gun is almost stock internally, though I intend to upgrade things as they fail, though it looks like that might be a ways out yet. Right now I've shot around 8,000 rounds through it with the way it's setup with no issues to speak of other than minor tuning. Piston is stock, still looks good and it's a 45 degree setup with SHS sears. I can't say if the JG would hold up to the same abuse lacking a better term as my TM has, hell I can't say if new TM guns will do it. Mine is around 10 years old now, but it's handled things just fine despite the horror stories;)
On the chamber I've read really good things about the AA Chamber but I haven't seen it in person, I suspect that will be my next upgrade sometime in the future just because everyone raves about it, must be something to it. When the piston fails I'll replace that but I'll likely stick with the 45 degree system rather than going to a 90 degree setup. Despite all the complaints my trigger isn't bad on the 45 degree setup even running a PDI 270(from memory, setup runs around 550fps).
So that's my experience, I had to see how long the stock parts last and so far I expected things to fail by now but they are still going strong. Last I had it apart maybe 1000 rounds ago, everything still looked perfect without any additional wear to speak of. So who knows how long it will go but I'd say unless your unlucky pretty long;)
Luke