One difference is that if you sign up (for free) with Avis Preferred you can skip the rental counter entirely. Budget has the FastBreak program, but it's not nearly as seamless.

Also, for a rental that far out, you might want to try booking a rental for the best (pay later) rate you can find and then tracking the rental with AutoSlash. By starting with that strategy this far out, you may be able to get a great cash deal and not have to burn potentially valuable Aeroplan miles on a car rental which is likely going to give you a crappy redemption rate.