my opinion, if anyone cares:
Castaignos is (was?) a better goalscoring threat. He hasnt really blossomed into full form since moving back to Holland, its hard to know if he's "lost talent" or just needs more time. Livaja, on the other hand, is a MUCH better all action forward. He will excel in any of the forward three roles - in a way that Castaignos failed so miserably at Inter. That being said, I dont think Livaja is - currently - a better goalscorer. I mean, those EL goals as you say are a bit overrated, and he's doing reasonably well but not exceptional in Serie A, I would expect a lot more from him before I hail him as a good goalscoring threat. But, as a forward, I think he's clearly a good player, in the more Pandev/Rooney at the moment/etc. He has a long way to go, though.
tl;dr Goalscoring threat - Castaignos (a bit). Overall forward play - Livaja (by a long way)