I have both and do different gigs with each.
The SABDDI sits on my pedalboard and warms up my tone on Electric Bass. The BDI21 is used as an 'always on' impendence matching/pre-amp/tone warming for my Double Bass. On this point, I tried a number of DB pre-amps, including the Fishman, but liked the 'warming' the BDI21 gives.
Tonally, they can do broadly the same things, although the settings are not the same to acheive it on both, if that makes sense. I have never had any problems with either, the DI from both is strong, although the BDI21 eats batteries faster and 'whines as the battery dies. Not a problem if you carry spares or plan ahead.
Construction-wise...one is made of plastic and the other is made of metal. My BDI21 goes nowhere near the floor, so this isn't a problem for me, but I would worry about it holding together under much foot stomping.
The Price/Quality issue is the main thing. The Behringer offers fantastic VFM, it is made from cheaper materials, AND it sounds fine. The SABDDI is more expensive by a factor of over 6:1, it is the industry standard, it is better built, AND it sounds fine. You pays your money, you takes your choice.
Shaker