In many ways, Engvall and Kerfoot bring very similar things. Both are strong defensive players. Both put up similar offensive production (at least with Kerfoot's offence disappearing this year). Both can play all three forward positions. And, on the negative side, both are soft and prone to the occasional gaffe.
But watching the game, Engvall always appears far more visible. Part of that is his combination of speed and size/reach, but part of it is also that he's also inconsistent. Kerfoot is what he is, night-in and night-out, whereas Engvall does have that extra gear, IMO. There is just no rhyme or reason as to when he decides to actually raise his game.
For those reasons, I think Engvall is the more valuable piece and the one we should strive to keep, but I also expect he'll bring back the bigger return.