RonIII

1. You can't

2. The reason is that Elites have bigger heads. The bigger the head, the easier it is to get head shots. Especially with the Battle Rifle and the Sniper.

Ultima(elite)

You can't get unlimited ammo in this game unless you are using vehicle guns (like the Warthog turret) or the Scarab Gun. Oh yeah, and on multiplayer, the sword has unlimited energy, but that is the only thing that does.

People generally choose Spartans because they have small heads located on top of the shoulders, which is rather difficult to hit. Covenant Elites have large, round heads that stick out in front of their body, so all you have to do to get a head shot on an Elite is basically aim for their torso.
People also have preferances on how the melee attack is performed (Spartans punch differently with the SMG than Elites do, for example,) but this is generally not a huge factor on why people are Spartans over Elites.

monkey298

Let me clear some stuff up.
First the head is the same size on both characters it just sticks out from the body from the elite. Second you can't get unlimited ammo with AR( action replay) you can only get special game types with that. You can get unlimited ammo with mods though, which are illegal on xbox live and they will get you banned. Oh, and another reason more people pick spartans more is because they can do sword cancels or glitches better. If you have any more questions you can contact me on xbox live my screen name is monkey298

Arbitor

You can't

Elites have bigger heads so it is easier for snipers.