I'd probably go with the Amex platinum. The Aeroplan platinum has a PP membership but it just covers the basic membership fee, you still have to pay $27 per visit. The Amex platinum has free PP visits for the cardmember and one guest (additional guests have to pay $27). You can also bring a free guest to plaza premium lounges in Canada and Amex Centurion lounges in the US.
Also, with Aeroplan winding down in 2 years, it may be wiser to collect Amex points and just transfer them to Aeroplan when you want to make a redemption (you can also transfer them to other programs).
If you apply for the Amex platinum card, be sure to use a flyertalk referral (
American Express Referral Offers (Read Wiki before posting).) since you will get 60,000 sign-up points instead of the 50,000 you get for applying from the Amex website.