Basically it's like asking: Is Williams' upside as great as Tomlinson's downside?
If I were you, I'd reject the trade because it's too early to write off LT just yet, especially since you probably drafted him early. Clearly the Chargers like Sproles, but that's no reason to dump off LT for a guy that most people didn't even draft (Williams).
Plus, did you see Tampas rushing stats today? I would stay away from that offense altogether!