Well they’re very different things. DV would preserve your video exactly as it is. This would be best if you’re going to put it back to tape or put it on a DVD or simply save for a future project. DV has predetermined settings. It will work out to about 13 GB an hour.
H.264m is a distribution format, usually for putting on a web site. You can scale its size and quality levels as low as you want in order to get it down to a particular size you want. (That size could be very small or very large depending on what you do.) This would be for people to view and would not be a master copy or something you could add back into a DV project later.