Spiller can be an explosive talent, but he's hardly been able to get in gear over the first four games. I don't know what's wrong, but when he doesn't have his wheels he's pretty ordinary. He doesn't have his wheels at the moment due to an ankle injury.
Jackson is one of my favorite Bills of all time (and I'm a fan since I can remember, about 1962) due to his determination and ability to get extra yards. For every run that Spiller breaks, Jackson has three tough runs that get key first downs. I don't hate Spiller, far from it, but we should look at what the guy is doing this year and not imagine he's doing magic tricks like he did last year.
When both are fully healthy, I like the mix that's been going on. When Spiller is a step slow, he shouldn't be on the field because he can't even get back to the line of scrimmage. When Jackson is a step slow, he still finds a way to get yards.