If all you are interested in is economy redemptions within NAmerica, then you are correct, Aeroplan will give you the most return for your Amex points.

However, most of us here are looking to book long-haul premium cabin redemptions without paying surcharges.

My point on waiting is that Amex MR -> SPG ratio has improved from 1000:280 to 1000:500 recently. It is possible for that ratio to get better as time progresses, so there is no need to convert to SPG now.

SPG gives flexibility in that you aren't locked into a specific airline alliance, and you can also use your points for hotel redemptions (cash & points can get into the 5-10 cents per mile if used at the right hotel).