I definitely think the terrain is more suitable to Ironman.
Also I find this match hard to judge because Smaug is hard to judge. He survived Molten Gold which is his best feat. That still doesn't mean he is going to automatically hold up well to modern weaponry much less the super advanced stuff Tony carries on him. His hide is hard to cut with Medieval weapons but that is not what he is going up against here.
Smaug's size could prove a problem but I do not think Smaug is stronger than the Hulk based on feats.
Also Smaug's breath weapon is nice but I do not know if it is stronger than your average flame thrower. Obviously Smaug's breath weapon is larger and therefore can attack larger areas but it's melting feats could be reproduced by a normal flamethrower I think or close enough. So I don't know how effective it will be on Tony.
I think I would go with Iron Man myself. I think it is too much a of a stretch to say that Smaug can just tank advanced weaponry without feats to back it up.