I'd have to say that the utility of a warplane is measured by what happens to is when the war that it has been in is over. B-17s and Lancasters served into the late 1950s, B-24s left the inventory as fast as they could be ferried to the salvage depots - by the end of 1946, there are no B-24s in USAAF service. Yes, the Navy kept its PB4Y-2s, but a Privateer isn't a Liberator.The Avro Lancaster could be modified to deliver the Dam Buster bouncing bomb as well as the Tall Boy, which was used to sink the battleship Tirpitz, and the Grand Slam to destroy U-boat bunkers. This was possible because of its cavernous bomb bay.
The B-24 Liberator was known for its long range enabling it to engage targets out of reach of the other heavy bombers. It closed the Atlantic Gap which contributed to turn the tide in this war theater in the Allies' favor and it could attack the Romanian oilfields of Ploesti from bases as far as North Africa. For the same reason it replaced the B-17 to cover the vast ranges of the Pacific.
The B-17 had the advantage of being able to drop its bombload from greater heights than the other two and it was a bit more rugged although its loss rate was about the same as the Liberator's. In the end the B-24 could do the same strategic bombing missions as the Fortress.
The Lib could also be produced more easily and faster being optimized for mass production.
So could one say that the B-17 could have been done without?
As for the performance differences - The '24 certainly did have longer range, but economical cruise speed (which for a bomber or transport is what counts) were actually pretty close - 165 mph for the B-17G and 170 for the B-24J - which is within the variation that you'll find in different airplanes of the same make and model. (Some are good, some are slugs)
The B-17 was better for excess power - it always has about a 5,000' advantage on the B-24 - all numbers from the Specific Flight Operation Charts of their Flight Operating Handbooks.