[Personal experience ... I had a BlackBerry 8800 on Rogers for 18 months or so (and older Blackberries before that), but a month ago our company switched to Bell. So now I have an 8830, which is more or less the same device, but actually has GSM for overseas travel, in addition to the usual CDMA.

Based on that month, though, I would personally pick Rogers every time, even with no overseas travel:

• Bell is dropping more calls than Rogers
• US data roaming coverage appears inconsistent
• Occasional problems dialing overseas (all circuits are busy)
• Occasional problems receiving calls (all circuits are busy)
• Can't put a call on hold and make another one
• $10/month to use GPS on Bell
• Bell voicemail usability is worse than Rogers
• Battery life on Bell (CDMA) is much worse than on Rogers (GSM)

All that said, if you're happy with Bell now, you already know what they are good at and what they aren't, and you won't lose anything by staying with them.