You need to look at your destinations. Will you need to go through a hub to get to most of them? If so, then you really have lots of options.

In the US, aside from quality of service (lots of very subjective opinions out there, so go with what you like) things are relatively equal. With 2 flights you can get anywhere in the US on NW, AA or DL.

As far as NWA is concerned it has the most flights to Canada. A lot of their flights there are on Saabs and CRJ, so no upgrades. If you travel to YVR, YYC, YEG, YUL and YYZ you are likely to fly Airbus or DC9.

DL and AA have limited service to the bigger cities in Canada, I do not know aircraft types but I seem to remember a lot of RJs.