With a "z", because the "b" is voiced, whereas the "s" is voiceless. It's perfectly natural for that feature to spread to the next consonant.
On a related note, I said the word a couple of times before I posted. I tried to make it an "s" but that made the "b" more of a "p". Again, down to the voicing.
(I believe there is an actual linguist on this board, and not just me, who took it twenty years ago. Maybe she/he can explain it better.)