The EM-1 was an outstanding locomotive, of rather advanced design compared to the N-5 and even M-4, since it was designed later with more "modern" technology. But I think it is pointless to argue which Yellowstone loco had the most power or the most this or that. You can never really settle such arguments - data is always a bit fuzzy and if one loco had more tractive force, another always seems to have had more power - or may, possibly have had more power - at 25 mph or was it 35? It never ends and really it never can.
I have a nice model of the EM-1. I like it a lot. What seems indisputable is that the M-4 and the N-5 were noticeably larger, heavier locomotives: they will have more "shelf gravitas" if nothing else, and so I would really like to have models of each, or at least one of the two, to add to my set of "big boys" that I display and occasionally run.
