We haven't really seen much of Finn as a trooper, so as far as who the superior trooper is? Rex, given he didn't go AWOL(I mean technically he did by removing his chip and thus not following Order 66, but that was the GE not the Republic). Yes, yes Finn has a backstory and all that, his training history and what not. But in an overall capacity, the superior trooper is Rex.
Physicals I'd say about the same.
H2H? Finn doesn't have any H2H showings as far as I know, so...Rex
In Blasters? Hmm....can't recall any blaster feats for Finn though he shouldn't be poor anyway, so gonna go with Rex on this too.
__________________ ''It is necessary that I should die for my people; but my spirit will rise from the grave and the world will know that I was right." The Almighty Führer