Both will deliver just fine, despite the B1 being a bit outdated by now. There are a few things you need to ask yourself:
1. Do I want to use my own (or 3rd party) IR's? As @dave_bass5 says, you can't load them in the Zoom. However it does have a few amps sims/cabs included. They are sufficient, but personally I prefer the Nux on this point. It just sounds better IMO, even without any IR's active. Note that you can have only 1 IR loaded at the same time, and you can't switch without hooking it up. The Nux is more a set-and-forget kind of preamp with an added drive.
2. Do you want to use effects or do you want a basic good clean tone? Both will do the latter just fine, but the Nux only has a single drive and no other effects. Not even a tuner, which is all present in the Zoom. If you play with effects a lot, the Zoom's an easy win.
Other than that, look at the feature you may or may not need: A separate headphones out, DI-out, do I want to use a PC to make major adjustments or not and such.
I would personally pick the Nux if you don't need the effects and tuner. If you want a one-box-that-does-it-all and can live without a DI and headphones out the zoom will do just fine.
As always, YMMV. Good luck deciding
