thanks so much for the advice. I was wondering if I was missing something but I'm not -- my problem is that I barely qualify for Elite i.e. around 40k miles so I don't get any threshold benefits from AC (I never get the brochures etc. since my address keeps changing and I've been losing mail - I had to call to get a duplicate card issued since I didn't get the original one). Moving to United gets me some benefits (25% etc.). Also since my flights are Can-US - both airlines are equally convenient. Also I've lived until April in Canada then until last week in the US so my country of "temporary" residence keeps oscillating so it's pretty much a toss-up. I do really like AC but a few more miles on UA expecially with the Star Alliance allowing my purely AC flights (such as YYZ-YOW) to be credited to UA also.

Anyways I'll decide at the end of the year since this year is almost over. Thanks for you help and advice.