Actually there is a Subway in Commerce Place, I think one on Jasper Ave and 100 St, one on 100 Ave and 107 Street, one in the Edmonton Tower and one on 108 St and Jasper, so that's five locations in the core.
So Subway 5, McDonalds 0. Also, I think Tim Hortons has at least three locations in the downtown core as well. Both have outlasted and I would say out competed McDonalds.
However, I agree a street front location some where downtown would be better that a food court for McDonalds, as it could get more traffic and have longer hours. IMO, 109 St would be a good spot.