This question has been asked, and answered, dozens of times on this forum. Spend a week reading the volumes of previous info.
You just enlisted. You have absolutely zero knowledge of what your job will be when you are assigned to your unit. MPs in my units work personal security (checking backgrounds for clearances). The MP unit I spent a year with in Baghdad worked prisons. The last NG MP unit I worked with was doing gate duty at Port Ash Shuaiba. None of those guys need a fixed blade for daily carry, and in two of the units they were specifically banned.
You are not going to deploy for two years, minimum, and at that point there may very possibly be no deployments. Even if your unit has been told they will deploy (which they know 24 months in advance) they are very likely to be cut in numbers or all together. We are expected to be out of Afghanistan in 2014 and they're drawing down. You won't be done with training by then.
You're a Newbee. You can go out and buy a big, honk'en, bad-assed knife and years from now you find it buried at the bottom of a foot locker or you can sit back, get through your training, learn something about the job you're going to do, and make a much more informed purchase.
Good luck.