It's complex now because CDMA is used by both systems! There's the 1x/EvDO version of CDMA that Telus, Bell, SaskTel, etc. use... and there's WCDMA that GSM providers use in their 3G networks all around the world.
2G GSM has some advantages over 3G GSM and 1x/EvDO, primarily in the area of battery life. 3G GSM gives slightly higher voice quality (to my ear 2G GSM sounds better than 1x/EvDO voice by a long shot, but some carriers, particularly in the US, use a codec that compresses the voice more than the one Rogers uses which reduces quality noticeably) and much higher data speeds, but for voice none of this matters. A 3G phone will give you the ability to use either service, which can be useful during times of great demand on the system, but otherwise having 3G is a liability for voice because of the reduced battery life.
If you want to be able to unlock your phone and use it anywhere, GSM is your technology. In a few years, though, everybody is going to be using GSM (probably 4th generation) so the point will be quite moot, but I love that I can pick my own hardware with GSM. I have a bunch of phones and I use all of them depending on my whims and needs of the day. I used an unlocked phone in New York City last week with AT&T, and got a SIM for Red Pocket Mobile that allows calls from the US to Canada without charging extra for toll, to use on my next trip. I like having that kind of control.