I don't really see any theoretical reason beyond personal preference, and the fact that Rosicky is currently fit, to keep Rosicky and not Diaby as far as their contributions are concerned.
Both are injury prone, both are marginal, both have the capacity to change a game of football in a fairly erratic way.
As to whether we can afford both, we could afford both and two or three more of the same if we wanted to, but we might as well let at least one of them go.
