To me, they're in similar situations. It depends on how long Jackson is out. 2-4 weeks, or longer I'd guess. So Spiller has the starting job for the time being, and maybe for the year.
Morris is going to start in the same time frame, in my opinion. I know Shanahan and his rotating backfield are a concern, but if he keeps putting Griffin in Shotgun and Pistol, I think he makes the most of his carries when the linebackers don't know what to do on a given play.
So, essentially, it's almost a toss up. Personally, I'd go with Morris. I think he'll be the most consistent based on his team's strategy. Spiller had a hellava first game, but I don't think Buffalo has enough weapons and that he'll be keyed in on more plays then Morris.
Also, roster would help in this situation.