I think this is a great question and it is one I have wondered, particularly since I freely admit I know very little about Morris other than seeing him in a couple games.
At the least, Maroney is a back unlike any we've had before. We saw glimpses of his NFL ability last year. I suspect, as people here are wont to talk about, that BB, as a master of maximizing pros and disguising cons, will devise a system to benefit #39. Perhaps this is what led to Corey's escape.
Any half-way decent starting NFL back is good for 1000 yards these days. In hindsight, Maroney's talent belies his relatively "late" 1st round draft status. In a workhorse system, he is capable of 1500 yards. He won't be needed in that way in this system.
The recent WR acquisitions points towards a more pass-happy offense, perhaps with creative flea-flickers, etc. such as we were introduced to last year. Having Brady at QB, with a bevy of capable receivers (!) of disparate tendencies opens up every corner of the field for exploitation.
Maroney may well be the twitchiest, quickest, most explosive back since Barry Sanders, Reggie Bush be damned. Used properly, he could be like trying to defend against a ninja when you're trying to stop the others circling you. The NFL doesn't fight one-at-a-time style like in movies.
If Morris can be faster, quicker, and just as strong as Dillon (would have been in 2007), the running game will be just fine. Of course, Dillon was a wily veteran. But as many touchdowns as he had, can you honestly say that when he actually broke thru the first line, damn, why can't he run any faster than that? Go Dillon, go! go! go!!
My guess is Maroney will get 1000-1200 yards, Morris will get 300-400. Brady will pass for well into the 4000's.