I'll do my best to keep this brief....but afraid it won't be (apologies)
I own and have gigged the B3 for quite a few years, and though I appreciate the feature set dedicated to bassists, there are two things that I could never get past: the limited 3 button footswitch design, and the fact that the modeled tones, though respectable for live use, seem to my ears to be lacking in depth and definition compared to the better (more expensive) modelers I own or have owned.
No knock really on the B3...fantastic at its price point. When the G5n guitar processor was introduced last year, I took a chance on it. The footprint is significantly larger than the B3, but not terribly huge, and well worth it considering how much more functionality/switching you get on the G5n. The big downside is that the G5n isn't geared specifically toward bassists, as is the new B3n (which I will own). I've punted, but been quite pleased using the HiWatt amp/cab sims for bass. Not a bad tone/vibe for bass at all, though quite midrange present.
Anyone who is pre-disposed to consider the new "n" series of pedal products to simply be an upgrade built upon what was established with the B3/B1Xon and the G5/G3 may or may not be thrilled with these new pedals. They really are conceived to be different conceptually, once you get beyond the obvious similarities. The "n" series is all about the upgrade in quality of the modeling across the board, and the algorithms have been re-worked from the ground up...all about speed and simplicity (WYSIWYG)...all about added capacity having up to 7 effect slots.
Having had time programming guitar sounds on the AxeFxII, I can vouch for the quality of the G5n guitar modeling and cab sims. Absolutely different leagues no doubt, but I've heard worse amp models in more expensive pieces of gear. Again, at this price point, G5n models are not too shabby, and (at least to my ears) contain more depth and detail in the amps, cabs, and drives compared to previous iterations and other modelers in the low price range.
Yes, they've ditched the XLR. They've also changed the "global" feature of the Looper and Rhythm modules. Instead, now, each is considered an insert effect within a patch, so you can't change patches with the looper/rhythm modules running and have them continue in the fashion they do with the B3. However, since you have a total of 7 slots maximum, you still have 5 slots left for "stomp" mode effects. Little consolation to Looper fans, I'm sure.
I suggest that you consider the B3n for the improvements in quality of the amp/cab/fx models, for the extra switches, and the extra fx slots. If you're wildly happy with the B3 and also rely on its built-in XLR, then the trade-off in sound quality and extra fx slots may not be worth it to you.
Speaking for myself, I will generally only be using the B3n for its Amp/Cab simulation and tonal/dynamic post-processing, and I'm excited to hear what the new amp algorithms sound like, as well as what ZOOM will be offering for future (downloadable) amps/cabs/fx. For many gigs, it will be used in conjunction with my G5n (which will provide the stomp effects...up to 9) when I need the flexibility of sounds. For simpler gigs which require little in the way of changing sounds and effects, I'll be using the B3n by itself. My road-worn B3 will be designated for home and backup duty.
A note about the XLR (or lack thereof) and how I personally am not bothered by this...I use only Full-Range cabinets for stage, thus my tones are fully baked with amp/cab modeling, and crafted to suit the needs of FOH. To account for stage acoustics, I run my pedals into the front end of a GK 1001RBII, and pull my FOH DI feed from its DI output, set to take the signal PRE the GK's preamp. I then use the tone controls on the GK to tailor the tone to my liking according to the needs of the stage, independent of the FOH feed. Over the years, this is what I've found to work best for me. A simple power amp with no tone capabilities means you only get "volume up" or "volume down", which is a bit stifling, especially when all you want to do is lop a few dB off of the high or low end bands, for example.
Again, sorry for the long post.