G3-SG1 (or is it G3-SG/1? IDK, I don't really care) barrel length is 469 mm. V2 for a winter gun... hmm, even I have concerns for that - though its just people telling me a V2 won't survive the milder winters here in Vancouver. My current box is actually the most reliable one I've had yet, TBH.
M14's hop up seems superior to me... that is, it was when it was shooting. I've got a 20 FPS difference between a TM M14 Socom and a CA/TM bastard G3, M14 shooting with a little more FPS and getting quite a bit more effective range. That being said, I can engage with decent effectiveness out to about 200 ft with the G3. M14 seemed like the BB's it shot were going much further... never got a chance to really test it before it broke down on me *sighs*. V7 box is supposedly really good, or so I've been told. I don't do work on AEG's, so I wouldn't know the specifics, nor have I been around long enough to really figure this stuff out yet.
As to mags, G3 mags are thicker and there are less choices (to waste your money on, lol). G3 full stock can easily accommodate bigger batteries - if you need more space, there are posts that you can easily knock out to create some more space; the M14 hatch into the battery compartment is smaller, though it doesn't matter to me as I'm on lipos.
That's all I can think of off the top of my head that might help? IDK, I'm tired and am totally not thinking straight................