After reading everything I could about the Tech21 sansamp bass driver DI (BDDI) and the para driver DI (PDDI) I decided to order both from musician's friend and try them out at home. I should state my ideal tone is somewhere in the realm of Robbie Shakespere, John Deacon, Tom Scholz (Boston's first album), Adam Clayton and John Entwistle ("Who Are You" and "Quadrophenia"). I used both into a combo with the eq flat as well as into a mixer with headphones.
My impression echoes what others have said on the forum - they are both great units but are different. (I liken it to comparing "blue" with "green" - neither is "better" just different) In particular, the blend and drive controls have very distinct behavior between the units. As has been said, the PDDI has more overdrive (approaching fuzz) and a darker blend function.
The mid control on the para is nice and useful but I found with a Fender Precision I didn't need it as much as I thought I would (and the BDDI could muster some mids after some tweaking). Before I got the units I thought the PDDI could do everything the BDDI could do and more, but I'm not so sure after my experiments. You could get them to sound similar but the blend and drive was so different imho that I wasn't always able to make that happen in all cases.
Since I found myself gravitating toward a similar sound on both units, the question then became one of which allows me to get the sound(s) I want. As I said, the units have different drive and blend characteristics. I found the blend to really thicken (darken?) up the sound on the paradriver in a nice way. I am not big on using overdrive with my bass (others do it well but it isn't for me) and found I was able to get more subtlety with the BDDI in this realm. The paradriver can get a nice fuzzy overdrive tone when pushed and the mid really helps here - for those of you who are into fuzz you might want to check it out. Again... not what I am going for tone wise.
If I were going to use one of the units for guitar *and* bass I suspect the paradriver would be the choice because of the overdrive and the mid control for guitar. However, I just found I liked the BDDI more when playing bass. I not sure why exactly (kinda like explaining why you like "blue" vs "green"), but the blend and drive (after trying many settings) was just more "me" on the BDDI.
I think you can see the punchline coming except that I returned them both... I decided I liked the BDDI more but wanted the programmable bass driver so I can have a "set it and forget it" rig if I use multiple basses or for different songs.
If you are considering the BDDI/RBI vs PDDI/RPM I suggest trying them first (after reading all you can). I don't think you can go wrong either way (Tech21 makes great stuff) but let us know what you decide if you do compare the sansamps.
My $0.02
