Welcome to FT, and yes, might as well milk the SPG Amex for all it's worth before the Marriott takeover. Right now the 3 to 1 conversion to Marriott points is very good.
Even if you got one of the Canadian Aeroplan cards, you would be earning 1 mile/$ spent (except for grocery, gas, pharmacy). With the SPG Amex, you will be earning 1 point/$, but when you convert to most airlines (UA being the notable exception), you can get 1.25 miles/point. For UA, you're best to transfer to Marriott points, then to UA miles from there.