I'd go with Pierre Thomas. Even though he's not as versatile as Reggie Bush, he's all but gotten the nod this year for a few reasons:
He's had an excellent camp, has gotten stronger, and is going to get more carries: http://www.rotowire.com/Pierre-Thomas-g ... -spnfl.htm
Reggie Bush isn't healthy, so Pierre is the much more stable back in New Orleans: http://sports.espn.go.com/espn/blog/ind ... y_football
Thomas is also going lower than his value would indicate due to the 'value' of his name, or lack thereof
Ryan Grant is a solid number 2. While he'll never reproduce his epic season from 2 years ago, he's in for a solid year after the letdown from 2008. There is the concern of Brandon Jackson taking his carries, as well as Aaron Rodgers's ability to run with the ball. The O-line is also getting older, which shouldn't work in his favor.
He does look to be a workhorse this year, and if he is explosive he has a shot at a top 10 RB season, but I think Pierre is the much safer bet.