The key differences are use, quality, location, and brand.
MLS has retail, hotel, office, and residential, all in one package. Such diversity in use is great planning for all day traffic and livability.
My own observation is that MLS is a clear notch above CP in terms of design and materials. No use in debating this one since the design is subjective, and none of us has the bill of materials from the architect.
Location? MLS is attached to the ACC, has direct access to Union Station, and the PATH network. You can go to a game, the office, shopping for miles, without putting on your coat. CP is trapped between the railway trench and the Gardiner with fewer connections to the city. You walk, or get stuck with the suburban inflow on Spadina.
And for brand, that's huge. One is attached to one of the best brands in Canada (even if they can't make the playoffs - I'm not going to defend their record, but have observed that the brand is solid - they are the most valuable team in the league). MLS also have Le Germain, Longo's, Real Sports, etc. CP has a Firkin pub, some dry cleaners, a bank? Big deal. Concord-Adex itself seems to be the definition of disposable brand.
And the last thing I would point out is the urban fabric around each.
MLS feels urban. Cross York Street, and not so much, so MLS really benefits from its proximity to ACC/Union/PATH. CP feels like vertical suburban towers in the park crap that we know does not work. Each of those towers should be surrounded with a MLS style podium.