Wanting to join the Marine Corps?

Well at first i had never given much thought to joining the marine corps, since i had heard alot of negative things said about it, and had talked to an army recruiter first. I decided to look into it and found myself finding myself more interested in joining the marines. Then my army recruiter had told me the Marine Corps doesn't guarantee mos's, i looked up what he said and it was true but saw that they guarantee occupational fields. Now is it hard to get MP field guaranteed in a enlistment contract as long as you get a decent score in the ASVAB?