From what ive read, after extensive researching online etc, there is good consensus indicating that the P30 is the back that produces the best out of the box images and requires the least effort to produce great results.
That means, the least Moire, color problems, etc. As close to a DSLR in regards to ease of use as it ocmes in the MFDB world.
The P30+ should be great at high iso too and the Phase backs are already excellent in long exposures and the + will probably just reinforce the fact.
So I would get the P30. It still has enough MP so that it still has and will have some separation between it and the 1Ds mk2 and upcoming Mk3. The mk3 will have close to 22MP! The P21 will probably still be able to produce better images than the unreleased 1Dsmk3 but still best to be above it in MP for marketing purposes.