I switched from UA to Aeroplan a couple of years ago. Fewer miles needed for premium awards where I wanted to go. Can also transfer Starwood points and Amex MR points 1:1. They also have a Bank of America MC so if you collect miles with a credit card you can still do it. But these days I just charge everything I can to the Starwood Amex card since they give a 5000 miles bonus for transfers of 20000 points to Aeroplan (and other FF programs).

But I guess it depends if you are looking to do upgrades or exactly what you plan to use your miles for.