I also use both sites, with a small side of Digital Press on the off chance that I find something that is really unusual.
And yes, each site does have its own pros and cons.
While GameTZ does have a nice matching system, good luck actually getting trades for some of the so-called "rarities". There are people with a nasty little habit of having items on the tradelist and being unwilling to trade them at all, or unless the trade is immensely in their favor. (ex: JoeBlow gets Final Fantasy VII, sailorneorune gets Snood)
The overvaluing thing extends to all sites; for example, the Amazon Marketplace is full of all kinds of delightful chaps who pulled their asking prices out of their respective bums. A local chain of stores is starting to follow this lead, ensuring that the copy of Tomba sitting at $75 will sit there until I TURN 75.
Both sites have a PM system with alerts. CAG's is a lot more obvious than a little letter icon.
GameTZ has a bit more organization on its trade boards (Console Marketplace for PS1 and newer console games, Portable Marketplace for GBA/DS/PSP/etc.), but if your thread ends up in the wrong place by accident, you are told very quickly by everyone and their mother.
The post feedback system has got to

ing GO. One thing that is nice about CAG is the lack of this system, meaning that some spiteful, gutless twit can't go around negging people they don't agree with for stupid shit. There is someone who goes around doing this to me because they may be thoroughly convinced that I am a communist. My values do differ from the majority - so what?
Also, another thing I've noticed is the public humiliation of any poor bastard stupid enough to scam someone (or multiple someones) here. People rally together to nail the scumbag to the nearest available wall. Most of the time, when this happens to people, they are met with "LOL"s and "sucks to be you"s.
GameTZ does have the Bad Trader Report viewable on the bio pages of the no-good jerks, but the offending trader can just drop off the face of the earth. Also, if these reports are removed, anyone daring to trade with said bastard may be none the wiser. I guess in either case it's just best to ask.
I've built up quite a few trades on GameTZ (95) as well as CAG (25) with good, honest people.
GOOD. HONEST. PEOPLE. The existence of these people is what makes all trading sites go 'round. So enough with the fighting already.
Can't we all just get along?