The benefit of pro rez over HDV is that it is not a long gop (group of pictures) format. HDV bundles frames together, that is why it has such a low data rate and takes up little room.
in your circumstance it doesn’t make much sense to edit your “offline” cut in pro rez. The data rate and file size is probably too big to run off a firewire drive.
However, if you plan on doing a color correct and laying your show off to tape, you will most certainly want to “re-compress” your show in the media manager to pro rez as doing any print to video/edit to tape with HDV causes FCP to have to “compile” all the frames, and this is an exhausting process.
sumation:
1. capture and work in HDV codec
2. finish in pro-rez