B24s are a little bit faster than the B17s. With the same bomb loadout and fuel load, the B24 will both outclimb and outrun the B17. The B24 also has significantly greater range. The drawback is that the B24 has very fragile wings and will break apart from stress and gunfire. Both aircraft are armed to the teeth, with the B24 having a slight edge in defensive armament. This is mainly because the nose and tail turrets have a greater range of motion than the B17.
Which is better? Neither is significantly better than the other, unless you are concerned with range. Both aircraft are easy to catch in fast fighters and will die the same if not defended properly.