The AeroGold and the Aventura cards are not the best (I still back Amex as the best of the cards), but they are very good backups for when Amex cards are not accepted. And until the end of the year, both the AeroGold (Gold / Infinite) and the Aventura MasterCard are free.
If you can only apply for one, go with the Visa. It is effectively identical to the MasterCard in accumulation (1.5x points per dollar for gas/grocery/pharmacies), but unlike the MasterCard, it transfers immediately to Aeroplan. The MasterCard uses frustrating Aventura points which can only be transferred in increments of 10,000 – which is not very useful. But if you already have an AeroGold VISA (and who doesn’t?), this might be an easy 15,000 points.