Both are great units but it really depends on what you want.
The Reddi sounds a lot like a warm and very clean bass guitar amp with a big bottom but the aggression in the bottom is in a different place than most bass amps I've heard. I have not worked with an ampeg b-15 so perhaps that is the issue as it allegedly sounds like one. Incidentally, I returned the REDDI i owned after one day.
I actually preferred the reddi on guitar versus bass and i strongly prefer most bass preamps (like the alembic f1x) and the sansamp BDDI over the reddi for bass because they add more color and have a smaller but more aggressive bass.
The demeter is not as big sounding as the reddi but its a nice translation of 12ax7 di. It puts the bass aggression where i would expect something with a 12ax7 in it and in different place than the reddi. Its not as amp-like as the reddi but still very nice.
I have - and love - both (the older verson "VTDB-2"). But they are totally different. The VTDB-2 has a very high input impedance and provides great clarity and articulation (but still has some "warmth" - depending on which tube you use). The VTDB-2 also works well with other acoustic instrument pickups (piezo or magnetic). The REDDI with it's much lower input impedance sounds much "thicker" in the low mids, almost like a miked bass amp.
It depends on the song/arrangement which I like better for my passive Fender Jazz Bass. I both cases I use a LMNOPRE to get a "healthy" level into my converters (I don't like the REDDI set to max. gain - to muddy for me, the LMNOPRE ist clean, fast and punchy).
Listen to the samples at Basstasters. You can hear the different charakter very clearly...
I recommend listening to the bass preamps on there as well (as opposed to just the DIs) if you prefer more color on your bass signal. Sadly most bass preamps are no longer made but there is alway used.