I think Gaara would win. He is stronger than Neji (especially when Shukaka activates, his demon) and his style is a bad match versus Neji's style. The best way to beat Gaara would probably be some sort of mind jutsu, that bypasses his armor completely. I do think that he will die later on, it would be nice ofcourse if he became a nice guy, but I think he will die to remind us of what monsters a cruel society can create.