The problem with VS things like these, like with two guys like Niner and Carter, is there's 50 million different ways that either of them could win. Might be wrong here since I only have a so-so knowledge of Halo stuff, but Spartan 3s like Carter are basically just really tough people with really good armor/weapons. Which is basically like any other Republic commando, Niner included. It's not like Master Chief VS Niner, where (as much as I love Niner) the Chief would win 99.99% guaranteed. And it's not like Master Chief VS Galen Marek where (as tough and awesome as Chief is) there's no way he could win against Starkiller's powers.
They're normal guys so pretty much anything could happen. Either one could get off a lucky shot in the first 2 seconds, the fight could go on for hours until both are out of all ammo and it turns into a melee fight. Even then, maybe Carter gets Niner in a choke hold and wins, or Niner just gets pissed enough that he beats Carter to death with punches. I'm not really against VS topics, aside from the ridiculous amount of dumb arguing and crap, but you can't really gauge people like this, and there's too many "Well maybe he did..." or "But what if he..." for normals.