I have a DMP3, so I am biased. But the DMP3 is very good for recording acoustic guitar IMO. I like it for vocals as well. A while back, somebody posted a pre comparison (recording acoustic guitar) between the RNP ($475) and the DMP3 ($175). The DMP3 fared very well.
It gets real interesting on page #2 when he re-records the piece and posts the results without identifying which pre was used on which take. On page #3 he reveals which was which.
In this next thread the same member compared the RNP, DMP3, and the VTB1 as a Bass D.I. Again, the DMP3 fared very well. The VTB1, not as well.
The dmp3 is a better value. And it probably sounds better, too. I never really exhaustively compared them, though, to be fair. I'd think the variable impedence function on the vtb1 might be a useful feature.