First off, both could be moving on, though I think both will stay with their respective teams. AA being RFA and Detroit needing vets (who can actually contribute) that can possibly play 2C.
Athanasiou - career 117 pts in 303 games (avg 0.39 pts per game)
Gagner - career459 pts in 844 games (avg 0.54 pts per game)
Athanasiou has the advantage of his youth and playing on a much better Edmonton team, but then Gagner will only be turning 31 in august (AA turns 26 in the same month) so he's certainly not over-the-hill yet. Imagine if both players have something like a 40 pt season next year. Will Holland neck himself knowing he gave up two 2nd rounder for a lateral move?