It does seem to be a difficult choice, but the bottom line is that if you are typically in a 4G area, then you can get either iPad. It only matters when you are in an area without 4G coverage - then you have to choose either GSM (Rogers/AT&T) or CDMA/EVDO (Bell/Telus/Verizon). Since you're looking at Bell, go with the Verizon model. At minimum, you'll get 4G with the Bell SIM card, and if you can get the setup and configuration right, you might be able to get 3G in Canada, although that seems tricky.
Apple has typically sold the GSM model in Canada, however Bell Canada does use EVDO for 2G and 3G connections, so the Verizon iPad 3rd generation might be more suitable for your particular use.
Note that everyone in Canada is moving to LTE, and both models support LTE as well as the various radio frequency bands needed for any given common LTE network.
For comparison, check out the technical specifications listed for the iPad 2 on Apple's Canadian store:
Versus the technical specifications listed for the iPad 3rd generation shown on Apple's US store:
Canada iPad 2 Wi-Fi + 3G model: UMTS/HSDPA/HSUPA (850, 900, 1900, 2100 MHz); GSM/EDGE (850, 900, 1800, 1900 MHz)
AT&T iPad 3rd gen Wi-Fi + 4G for AT&T model: 4G LTE (700, 2100 MHz)1 ; UMTS/HSPA/HSPA+/DC-HSDPA (850, 900, 1900, 2100 MHz); GSM/EDGE (850, 900, 1800, 1900 MHz)
Verizon iPad 3rd gen Wi-Fi + 4G for Verizon model: 4G LTE (700 MHz)1 ; CDMA EV-DO Rev. A (800, 1900 MHz); UMTS/HSPA/HSPA+/DC-HSDPA (850, 900, 1900, 2100 MHz); GSM/EDGE (850, 900, 1800, 1900 MHz)
Some online reports have suggested that the LTE (4G) SIM cards from AT&T work on their Verizon iPads, which indicates that it's likely that on 4G networks it doesn't matter which iPad you buy - it only matters when you have to fall back to 3G or 2G away from population centers. Since Bell Mobility in Canada uses CDMA/EVDO for their 3G coverage then you'll likely have better coverage if you purchase the Verizon model in the US - but it's not a guarantee that you'll even be able to connect it since it's not SIM card based, as GSM is. You'll need to provide the IMEI to Bell Mobility, and hope that it's not locked to Verizon in the cellular chipset.
I believe you'd have better luck choosing a GSM provider (Rogers) in Canada and using the AT&T model, switching the SIM card as necessary between the US and Canada.
You might find the following useful: http://en.wikipedia.org/wiki/List_of_Canadian_mobile_phone_companies
It indicates that of the three providers Apple supports (Bell, Rogers, and Telus) only Rogers has GSM 3G coverage - which is interesting because that means that Bell and Telus customers don't get coverage outside their LTE areas.