HDV, being a 1440x1080i format has a 25Mb/sec datarate (like DV). Why is the 60Hz version and the 50Hz version not different after compression?

MPEG2 can be reduced to ANY reasonable data rate. The more you choose to reduce it the worse things look. Since 25 Mb/s is available on tape, they chose to use it all.

Thanks. Agreed. So 60Hz HDV has about 20% higher compression as compared to the 50Hz version. Right?

Well, yes and no. They use a different size Group of Pictures. And the faster frame rate means that you're getting more temporal resolution from the US/NTSC/60hz version.

50Hz uses a GOP of 12. 60Hz uses a GOP of 15. So you can't just directly compare them and say that one's more compressed than the other, because while the NTSC/60Hz version is obviously more compressed (fitting 60 fields in the same bandwidth that PAL/50Hz only fits 50 fields into) the longer GOP allows for more efficient compression so... is it really "more compressed"? Early Z1 users did some tests and decided it was pretty much a wash, both looked about the same compression-wise.

Agreed Barry but the amount of compression being the ratio of the original datarate vs the compressed datarate is higher for the 60 Hz systems. Image quality wise, the peformance of the HW codecs are of course involved.