Paradriver vs. normal sansamp d.i.

I assume you're comparing the SansAmp BassDriver DI stompbox to the rackmount version. I haven't used the rackmount myself, but I'd assume you'd have more control over mids (since it has a mids knob), if you care about that sort of thing.

The funniest thing to me in the manual for the stompbox was where they say something like "to adjust mids, simply turn up or down the treble and bass knobs accordingly."

I love the sound of the stompbox, and most modern active basses have enough EQ controls onboard to get your mids into shape before you hit the DI stage anyway. However, if I was making a long-term purchase for my studio I'd go for the rackmount, it's worth the slightly extra money for rackability and an extra knob IMO (even if you don't use it much.) I plan to purchase one this year.
 
Hmm yeah I just realized you were asking about the regular SansAmp (not the BassDriver) - what instrument are you primarily going to record with it?
 
I own both the Bass Driver DI also known to bass players as the BDDI, as well as the RBI. The one that Geddy Lee uses. He doesn't even use cabs any more. Keep in mind he uses an Avalon U5 DI and a Palmer speaker simunlator. I have been very happy with the BDDI, but the RBI is a bit more versitile. It has more hardware options when it comes to recording and includes a MID knob, unlike the BDDI.

The RPM is more like the Accoustic driver SansAmp pedal but in a rack piece and with a semi-parametric MID. Some bass players will buy either both the RBI and the RPM, or the BDDI and the Accoustic Driver for more options. I have yet to won an RPM but that's because I still belive in a real amp in addition to other bass gear and I don't want my rack to be the size of lead guitar player's.