I would be a Spartan. Why? Well, the name Spartan sounds pretty badass to me. Plus being taken at the age of six and being raised as a warrior seems like they would have it pretty rough, and in turn, they turn out to be one of the finest soldiers in the UNSC. Plus, the Spartan's have sheilds, enhanced speed, sight, smell, hearing and strength wich makes them basicly untouchable. Plus the fact that their armor is better than Marine or ODST armor. And John-117 DID drop out of a HEV pod, so you really can't say a Spartan isn't as badass because they have never done so.
Last, I would have to say I would be a Spartan because we have cool armor permutations. Well, not really a reason WHY I'd like to be a Spartan, but anyway, Spartan soldiers are pure win. Not to say that everyone else fighting for humanity isn't. I just tend to be more of a Spartan guy. That's it.