I will reserve judgment on who is the better front woman, until I actually see Floor perform @ PPVIII (I've never seen AF play live before). I saw Tarja w/ NW at PPIV, the End of an Era DVD and Youtube videos, so I've seen how she is as a front woman. But, from what I've seen of Floor on Youtube, Tarja looks like she'll have some fierce competition.
As far as who is the better vocalist, it's Floor hands down! The power and heart with which she sings and her dynamic (changing styles between a rock/metal voice and a classical/operatic voice) put her far ahead of Tarja IMHO! Sure they both have great vocal chops and beautiful voices but, Tarja doesn't sing belt out a song quite like Floor and her style is one-dimensional (only operatic). I'm a fan of both singers' vocals but, there's no question in my mind as to who is the best. In fact, if anyone doubts if Floor can sing the operatic stuff as well as Tarja, check out AF's NW covers and see how Floor nails it, as if it were Tarja singing.