I'd think the B-29 would be more likely - availability is just as high as the B-32, plus availability of support (as already mentioned). Another point would be interoperability with the USAAF and even the RAF (B-29s in RAF service through to 1954) - standardised kit, as it were.
However, there is a political angle that may prevent either. Building Lincolns is as much a political action. Firstly, they're still quite aligned to Britain in the immediate post-War era, so favouring a Commonwealth product isn't too much of a surprise (the licensed production will aid the British who need the cash - pretty good diplomacy). Second, there's prestige to building your own hardware - "look at how successful Oz is - we build strategic bombers". Third, and probably most important is that building Lincolns rather then buying cast offs means skilled Australian jobs, particularly in the immediate post-War period.
So even though the surplus kit might be better in many ways (enough that even the U.K. operated B-29s in the early 50s), surplus kit doesn't provide the jobs or prestige that home built, shiny new kit does.