That's a tough one. It's a very lateral trade IMO.
They both play on #$%$ teams that have no other weapons besides themselves. One is a rookie, the other a seasoned vet.
Trent is the real deal, and barring injuries he'll finish as a Top 7-10 RB at the end of the year. MJD lead the league in rushing last year, and it doesn't seem as if he's slowed down at all this year. The guy is an absolute beast, and he's only 27(although he has a lot of mileage).
Like I said. It's a lateral trade, and personal preferance.