All of that is true, as far as it goes.
But it fails to tell the entire story. The B-17 did not serve in Europe in numbers until long after the Lanc. ( First flight in Jan-'41, Service in Oct-41, 1st mass mission in Dec-41) (1st combat ready B-17E in Sept-41, 1st large mission in May-42.) At the introduction of the first full B-17E in to squadron service, there were almost two thousand Lancs delivered for service. The first B-17 variant in numbers like the Lancaster had in 1942 was the B-17F, only 3,400 built. By May 1943, the B-17G, the definitive version started to see squadron service in late 1943.
Finally, because B-17s were also kept in the states, sent to the PTO and North Africa, there were never quite as many B-17s in England as Lancasters'. More Lancs for more months equals fewer missions because of serviceability issues.