Well is better that i thought,not great but i manage to create some very useful sounds.The stomp boxes are very close to real ones(i test them almost all in the past),and some are better.If you are a guy that can play dirt sounds out of stomp boxes in to clean channel amp you are fine.Ofc there is not traditional sounds like Boogie,Marshall or Vox but you can have all the major dirt like Overdrive,Distortion and Fuzz with the B3.The trick is to not use the cab emulators of B3.I find that the best results is when you use 20-40% of the mix cab.The EQ of stomps and amps works great.Just keep all the Balance knobs to 100%.
Ofc all the other effects works like the G3/G5.Reverbs/delays/fazers,synths/compressors etc is same between the units.
So if you have to have only one unit you can get the B3 with very good sounds but not the best.
If i was you i buy the MS-100BT witch is for both instruments(there is some bass amps and effects in the Zoom market for free) and one passive cheap DI.The cost is about the same for both solution.
All the above is my opinion and i suggest you to try it for yourself.
Good luck