Is the DI for bass? Keyboards? Guitar?
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.
Again this boils down to a matter of preference.