I had the Voicemaster and sold it. The compressor and effects where worthless. However, the preamp itself was maybe a very slight step up than a DMP3 but only for vocals and it was not that versatile. It has this slight Neve warmth thing going that makes it nice for some vocals. As for instruments, it was unusable to my ears as the warm sound colors things too much. So used with an RNC this would be slightly better than a DMP3 but only on vocals that needed a warm sound. If you need to record instruments too then move along.
If your main need is vocals or Bass DI, maybe guitars, then the Voicemaster Pro version is a worthwhile stop. It is my main vocal channel and has a really great sound - except for the compressor which I would not use on vocals at all, only for instruments like bass or guitar. I use the RNC with the Voicemaster Pro and am getting excellent vocal results. It goes from warm to clean if I need it and is more versatile than the original version.
A DMP3 and RNC combination has a wide range of use too. I always track acoustic with the DMP3 because it has a nice midrange emphasis. I have not gotten any stellar vocal tracks off the DMP3 but have made some very useable background tracks with it. Here is where the RNP would shine over the DMP3, for main vocals and drums. However, the DMP3 is better on acoustic in my estimation and makes a great starter all around choice in conjunction with the RNC.
Voicemaster $295 used on Ebay + RNC (You would need this) $179 = $474
DMP3 @ $199 + RNC @ $179 = $378
Voicemaster Pro @$550 + RNC @ $179= $729