Spoiler Below There's a way to tell wether you'll kill Zelos. When the party is going to leave Flanior, Zelos says he'll do something at the tower(Forgot). If Lloyd asks, "Can I trust you?", say bye to Zelos. If he says, "I trust you.", Zelos will not die.
I prefer Zelos over Kratos, personally. Despite not having Judgement, he's pretty badass.