Had the B3 and currently using the line 6 M5 with mission engineering expression. Which is better really depends on the needs. I like the modulations, reverbs, delays, octave and pitchglide(whammy)on the M5 better. (Hate the flanger). Expression control is also better than the B3. For everything else, zoom B3 kills it.
Used the B3 in the past for acoustic gigs wherein I go direct, using good headphones as my monitor. Amp sims are outstanding.