I have the P21 (not plus version) and the H2. I'm really happy with the quality of the files and the capture speed. I shoot mostly at ISO 100 so high ISO is not an issue, but I have made some test shots and they look quite ok.
I used a loan-P30 while waiting for my P21 to be delivered and liked the file quality alot but not the file size. Justed seemed to much for my needs. So the P21 is quite enough for me, making A3+ prints without any problems.
As far as moire issues I have seen some in clothing patterns, but its really not that bad. Not like the examples from P25 that I have seen. The moire in the P21 files are quite easy to remove in post.
The only major draw-back is the screen, but that goes for all the non-plus backs. It's really horrible, there is no other word for it. And no, I don't care that it is a little better to use in bright sunlight, it is still not useable for the lack of contrast and detail.
I have no experience with the Contax system.