seriously?
i can name lots of differences. Private miiltary contractors have largely gone unnacounted for their war actions. they do not follow the rules of war. they have only recently begun to be prosecuted for their actions, and they seem to get off with quite a bit, which is no surprise since there is a lot of money changing hanRAB between the government and these companies. people like you don't seem to care about their deaths or actions.
should i keep going?
as for the volunteering thing... if you think the only bad part of iraq is that 5000 american soldiers got killed, you have a very narrow view of the war, and of the world in general. replacing the (at least partially) accountable american military, with a (largely) unaccountable private army is irresponsible and as far as i can tell, a political tactic to divert attention away from a war that's still going on.
but now i'm wondering. what do YOU think the purpose of replacing a public military with a private military is?