A few things make me think it's Paul...
Firstly, the 'aaahs' and Paul's section seem to be on the one take. You can hear him take a deep breath right before it. Plus, although the tone of the voice does have a Lennon-type presence, it's very much kept in tune, something which Lennon wasn't great at with the higher notes (see I Am The Walrus). Basically, it has more aspects of a traditional vocalist, rather than that of a snarling, raw Lennon.
I really think it's Paul and I'd pay to find out!