Actually, HDV takes up no more space on a hard drive than DV. This is not true of DVCPRO HD. The following is from the Apple Final Cut Pro site:
"HDV uses MPEG-2 compression to achieve a maximum video data rate of 25 Mbps (megabits per second), which is the same data rate as DV. This means you can fit the same amount of high-definition video on your scratch disks as you can when using DV."
As to whether DVCPRO HD is better than HDV, it's a lot more complicated than Tanakaskier implies. HDV is a compression codec, just like DVCPRO HD. Compression codecs find similiarities inside a frame (intraframe compression) and delete the duplication to free up space. Unlike DVCPRO HD, HDV also deletes the duplication in different, consecutive frames (interframe compression). This, technically, allows HDV to have the same quality image with a smaller file size and data rate.