Good question Shareholder.

I have been at school in the States for over three years now and have stayed with Aeroplan because that is where my miles have been accumulating. As a marginal frequent-flier (maybe 30K on all airlines this year), I don't want to split up my miles among too many programs (particulary two or more Star Alliance programs).

I can also get a Star Silver card at 18K with Air Canada, which will get me E+ seating on UA (a real benefit).

If I end up staying in the States after I finish school, I will more than likely switch my membership over to UA.