It's basically comparing apples to oranges...
Mississippi River is a Virginia blend with some Orientals, a touch of Latakia and Perique. It's often listed as an English blend, but it's dominated by Virginia's.
Plum Pudding is a Balkan blend with your standard Latakia, Oriental/Turkish, Perique, Virginia, with the addition of Cavendish and black Cavendish.
Both are favorites of mine. I have a pipe dedicated for Mississippi River, not because it ghosts, but because it smokes great in that pipe. If I had to pick one over the other, I'd take Mississippi River. That's my preference though.