Melo out of college is probally, well definitely, more refined than Marvin, but this is a different draft. In any normal year Melo would have been, well, should have been the consensus #1, this year there is no consensus and only a group of 3 who seem destine for the top 3 picks. Marvin showed glympses of greatness all year long. He did struggle in games 3-6 in the NCAA though.
Marvin's strengthes are his athletics, his shooting ability (2 and 3 pointers atleast NCAA range), his height and size and his age 19 I believe. As we all know the high schoolers in the NBA are dominating now, and Marvin is basically a high schooler, so the pick wouldn't suprise me.