ODST, definetely ODST. Why? Imagine, you're going down in this pod, which isn't really spacious. It's getting hot, hotter than you and your gf in a bedroom. Temperatures get up to i dont know like 50-60 degrees? You hit the ground, really hard. The doors open, the air outside isn't really cool, but at least its not as hot as inside the pod. Bullets, frags, everywhere, you start running and shooting in the general direction the bullets come from, but you will have to find cover. One hit could mean a kill, reminder: you're not a giant tank with shields and enhanced speed, strength etc. It just AWESOME to be an ODST. They actually have to fight for real, form up in teams, not go on a solo-killing spree. They are not individually as talented as the Spartans are. They're tiny humans attacking giant brutes. And though they might not realise it (or do) but both the UNSC and the Spartans need the ODST. They performed formidable in operations which involved both Spartan(s) and ODST. They are the badass human troops, who gained their skill not by taking steroids, but by sole training. Also, it's not like you just sign up, start training and you become an ODST. It's just like todays SF's: only the best are allowed. Just my 2 cents
-DD