Having had both & having upgraded both, I feel I'd stay with the CA33.
No body creaking at all, unlike the G3. All metal vs plastic out of the box. Able to use a silencer without needing an adaptor. Good if you want a longer barrel or just to quiet the gun up.
CA33, I tried hard to make it jam, wouldn't do it. The G3 jammed several times.
CA33 mags bit scarce, but just get in on a group order and your all good.
"CA33Es have those ugly ass mags..", those mags actually have some character. They also fit the most common .223/5.56 mag pouches.
Now for stocks, both can take whatever your imagination wants. Folding UMP, MP5 Slither, PDW Folder, M4 Collapsing & even tactical sling end caps. (Requires a little do it yourself but it can be done.)
Both are V2, so upgrade parts are a plenty, IIRC the hop-up's are the exact same on them as well. M4/M16 type. Pretty decent, not as good as say the AK or M14's but it does the job well.
Uses all the same optic mounts, so their interchangable.
As for the weight of the gun, I found it to be very managable and I'm more then capable of wielding it no problem with one hand.
Anywho, for overall best opinion. CA33 hands down over the TM G3/SG1.
Have any indepth questions about either of these guns, feel free to private message me if ya want.
Want to see pic's of them, then check them out here.
http://s109.photobucket.com/albums/n.../Airsoft/G3A4/
http://s109.photobucket.com/albums/n.../Airsoft/HK33/
I started with a G3A4 & added the full stock later on to make it an SG1. In case your wondering. The only difference between the two is the stocks and battery wiring configuration.