Seriously doubt either will be FF. If the mirrorless is APS-C, that would be good. If it is four thirds, that would be disappointing. If it is APS-H, that would be great. If it is FF, that would be groundbreaking.
EF mount via adapter would be the best you could hope for as well in that regards. Doesn't make sense to buy a slim camera you have to put long lenses on.
Canon's pancake lenses need to be sharp and fast. That will be the key (f/1.4 to f/1.8 primes and f/2.8 zooms)