We might have a slight advantage over Forest if Smith starts.
If their team is similar to the one that played up there last time, they may be a bit apprehensive of how they will play against Smith. Anyway, it's just mind games. Let them worry about him.
Smith should get at least 20-30 min on the pitch. As said, bringing him on with 15 to go is not usually long enough to make an impact.
I would have Ben start against forest or if not at least half hour from the bench.