I would only replace the bad caliper, and not waiste my time and money on the good one. But DO get a new full set of pads, which I am sure you already know. You will soon know if that was the problem or not. I have heard of other problems causing the brakes to pull, including collapsed brake line, and obviously front end problems. I would look into the brake line, and maybe just to be safe replace both sides, as I have heard this is a very common problem. I am having the same problem but have not done anything about it yet.