Long time lurker, first time poster...
I own both the DMP3 and the VTB-1. They are very different preamps and actually compliment one another quite well. The DMP3 is clean and quiet, although perhaps a tiny bit sterile sounding to my ears. It has a wide gain range, so it would work well for any mics that need a significant amount of gain. It does not have any inserts. It has direct line inputs, but I've found that it sounds a bit wimpy as a direct box. Meanwhile, the VTB-1 is also fairly clean and quiet, though not quite as uncolored as the DMP3. It has a tube blend feature, which allows you to dial in some warmth into your signal. (Note: my VTB-1's tube has never worked properly, so my "tube blend" actually acts as a compressor. I've never bothered to fix it because, frankly, the compression is fantastic.) It has a crazy amount of gain - a total of 72 dB. I've found that its self-noise is very low, even with the gain cranked. On the back, it has both 1/4" and XLR balanced outputs, which is a nice bonus at this price point. Unlike the DMP3, the VTB-1 has an insert point. To my ears, the VTB-1 sounds much "bigger" as a direct box, but keep in mind that the extra compression on my box might be serving to big-ify the sound.
I have
an Audix i5 mic as well. I currently have it set up on a snare drum running through the VTB-1. The sound is accurate and punchy. I'm currently using the DMP3 for a couple of my toms, which are miced using Oktava MK-012s. The sound is very accurate.
In my opinion, you can't go too far wrong with either preamp. The DMP3 will probably get you a cleaner and crisper acoustic guitar sound, but the VTB-1 would improve the direct sound that you get from your guitar and bass. In the long run, getting both preamps would suit you well; you could use the DMP3 for overheads and the VTB-1 for the kick, for example.
I hope that this helps!
- Jerfo