Both are incredibly sweet cards. As already posted, they're about equal in value, it just depends on your preference.
Pros of trading: a). Soulfire seems to be declining in value still, and may be easier to trade back for in a couple of months (someone please correct me if this seems inaccurate).
b). Athreos is no longer on the print run. Therefore he may get harder to find.
c). From what I've seen in a friend's deck, Athreos is a card that is so mean, he kills small kittens in his
sleep. =P
Cons of trading: a). Soulfire has a good chance of rebounding in value if he finds a more permanent home in standard. He still has a while before he sees rotation!
b). Soulfire does tend to fit into a greater variety of decks, whereas Athreos is pretty focused on one specific color pair and deck style.
c). Soulfire is, to put it simply, 100% crazy amazing. Just his uniqueness alone makes him awesome.
I've probably talked too much for this one subject! But good luck figuring out the trade.