I’d recommend offline at DVCPRO-HD. Small file sizes, the G5 can readily handle it, it will look great for editing.
Then recapturing for your final will be very easy – no transposing of timecodes. Also, depending on the intensity and production level of the project, you might be able to deliver DVCPRO-HD laid back to HDCAM for final. That’s a network quality control question, but a lot of them will accept it.
I don’t think a dual G5 can handle ProRes. It’s very processor intensive. Having said that ProRes HQ is phenomenal – you’d be very hard pressed to tell the difference between it and uncompressed HD.
Personally I wouldn’t sweat the move to FCP6. In many ways 5.1.4 is more stable and less buggy. I did a lot of HD work on my G5 with 5.1.4 and it never let me down.