With Aventura, you're stuck to CIBC's credit card. You need to keep a credit card account open, and start paying an annual fee, to keep your miles useful.

With Aeroplan, you don't need to hold onto a card. If you cancel all your cards, all your Aeroplan points stay active. I have 4 Aeroplan cards right now, and I don't expect to keep any of them more than a year.

Here was my experience with Aventura: churn and burn.
I got my 20,000 signup bonus, and then converted all the points to Aeroplan last year (just in time).

Almost all of my points accumulation, for both Aeroplan and for Aventura, is through signup bonuses and promotions like the Aeroplan Star Challenge. So, for me, it's not a question of which is better, but how to maximize what I can out of each card.