I had read that Jamison's knees are shot and he won't be playing more than 25 minutes a game for the rest of the year. Diaw might be a bit more inconsistent in his production, but barring a trade you know his minutes will be there.
Diaw would at least land you the coveted combo of threes/steals/blocks, albeit with inconsistency. Jamison just doesnt seem to care, so barring a trade that moves Diaw somewhere else I think he's the better option. Every player rater under the sun would agree.