I don't know much about Giddens' game, but I do like Chalmers'. He seems like a guy who is a perfect backup point guard. He can score, pass, and shoot, and he makes things happen on defense (although he gambles a lot).
Now, if he was on the Celtics, I doubt he'd be seeing the time he's getting in Miami. However, from what I've seen, I'd absolutely take Chalmers here.