For the current 2.5G digital networks CDMA has better phone quality then GSM/GPRS hands down. The actual phone used certainly will affect this too though so you could still get a bad CDMA phone that sounds worse then a great GSM phone. Each has different coverage in different areas so you should look into where you want to be able to use it and which has the best coverage for you. CDMA phones might be a little more expensive then GSM phones due to the Qualcomm chipset.

iDen is another network that allows for the push to talk feature on top of phone/data.

Bell = CDMA
Telus = CDMA (I think)
Rogers = GSM/GPRS
Mike = iDen (I think)
I don't know offhand what Fido is.

I use a BlackBerry on Bell but for consumers the data rates here in Canada and the US are still too expensive for any full blown email sollution in my opinion.