From what I've read and heard from talking to a pilot who flew both B-24's and B-17's in the war, the B-24 was faster, carried more, and had a longer range. The B-17 was easier to handle (lower control forces) and might have been able to take more damage.
In the war, there were more B-24's than B-17's built (about 18,000 and 13,000 respectively).