I think you should go with whatever suits your character best. But, in my head, I feel that when someone wets themselves it's usually as a result of a scare, usually an immediate scare. But when someone vomits for this reason, it's usually as a result of seeing something that they didn't want to see, or something like that. Personally, in this situation, I like vomiting because it's a more violent way to expel something in response to a violent situation. But that's just me. Bottom line: do whatever suits both your character and the story.
I think it would make a stronger impact if his reaction wasn't a disgusting physical one. How about his eyes spread wide open, his mouth agape in shock and his breathing is reduced to labored gasps?
