Toshley is the safe choice. Decent stats and the spare parts can sometimes be useful.
Spells are more scarce in arena, so Loatheb typically just ends up a vanilla 5/5. Not terrible, but not particularly exciting either.
Deathwing is total YOLO.
I was leaning that way too. Went with Toshley and am 3-0 so far.
Deathwing would have been soooo bad with this since I ended up having a slower deck.