OP's Q was which was best MPA, Neptune or Shackleton...and the A is of course a lemon. RAF believed long loiter over water was better on 4 than on 2; Liberators, which with ASV had closed the "Gap" in May,1943 and thus won the War, had to go in 1946 (or be bought and supported with scarce $), so we retrod Lancs. Pesky RN kept niggling for Maritime Air and even to Sandys/1957 found some Ministerial support for the evident logic (copy USN, please). The only reason feeble RAF procurement budget, 1948 took on Shackleton MR1 was that Tudor's failure meant something had to go into Chadderton as bridge to Vulcan. Avro and the entire military procurement process dribbled the programme...and then came Korea.
Under MDAP US loaned us (I have 52; others say 53) P2V-5/7 and part-funded 69 Shackleton MR.2. US did that (UK/France) in other cases of evident duplication (Noratlas:Packet), taking the view that any kit was better than Front Line emptiness. After July,1954 UK's priority into US taxpayers' pockets lapsed, so we dribbled Shackleton MR.3 as we returned MR P2V-7s. Much Coastal kit was US, common to P-2/3, either free on MDAP/MSP, bought, or licenced (e.g Gulton Industries Autolycus "sniffer"). W-34/Mk.101 Lulu NDB was taken under Project 'N', USMC custody at St.Mawgan/Macrihanish/Sigonella, shared access with RNethAF/USN, for inter-operation under SACLANT. There would have been no point in incurring expense/distraction of AWE to do a UK-solo NDB ex-fixed airfields. We did so from 9/2/1973,WE177A(NDB) for deployment at sea.
A to Q is: if you accept 4 beats 2, then...there's your A. If not, then inasmuch as more P2Vs flew longer with more Users, then that must have been "better".
Last edited by tornadoken; 3rd Jan 2013 at 18:51.