WRT to earning rates, once you hit an elite level with AC you get 3 different choices of benefit packages (search the AC forum for more). One of those is 50% bonus mileage on AC flights plus 25% bonus on BD, LH, LX and NH. If you are going to fly on AC a fair bit then it makes a big difference to earn rates. If you hit Super Elite level the bonus rises to 100% on AC, not sure for the other airlines.

The other advantage Aeroplan has over UA is less restricted award availability, for *A airlines anyway, and less restrictive routing rules. This, to my mind, is a major advantage.