Simply put, the B3n should be a much better sounding unit across the board. I qualify this statement since I've only heard online demos, but based on my use of the G5n on bass, I can say with certainty that the quality of the amps, cabs, and drives are definitely an improvement. This also seems to be the consensus among guitarists who own the G5n as previous owners of the G5 and G3 series.
A cabinet IR is nothing more than a digitally captured "tone footprint" of a speaker cabinet as captured by a microphone. The real world analogy is this: in a recording studio, mic up a speaker cabinet with a Shure SM57 (for example), and listen to it in the control room. A digital cabinet IR is more or less the equivalent of mic'ing up a cabinet. The considerations are the same: what mic or mics should be used? What distance/angle/placement? In this respect, the number of possible IR representations for just a single speaker cabinet are nearly limitless, and the same techniques used in a recording studio will yield an almost identical result in a digital IR capture.
One huge benefit for bassists is that IR captures allow you to make "distance mic'ing" of a cabinet portable. Not many bassists who mic their cabinet are going to request a mic 2 feet away from it to send to the FOH and Monitor mixer. But in the studio, adding some space between a mic and a cabinet can be very beneficial tonally.