I have a B3 with an expression pedal, a MS-60b and a MS-70CDR. The B3 is a far better unit that the 60b. It's more versatile, controllable, editable and has a lot more FX's the choose from. The output from my B3 seems to upset my 60b and I have to dial down the B3 a lot...plus I can't go B3 stereo out into the 60B...where as I can with the 70b. It does stereo in and out. I also don't seem to have the same input gain issues with my 70b. It's a more focussed pedal, and it has a really nice array of chorus, verbs and delays. It's got more options for each effect type. The B3 works really well with the 70b and it pretty much means I can sans amp, dual compress, plus another FX (octave, synth...) on the B3 and then add Chorus, delay and verb on the end of the signal.
I've found the dirt options on these pedals to be fairly good....but very influenced by the input gain. The over driven amp models and Sans amp model seem to offer me the best sounding dirt as opposed to the OD / muff sims. I'm not a big dirt player, but if I was, I'd probably look for a better dirt box to put between my B3 and 70b.
One gotcha...the patches on the B3 read left to right, but on the 60b and 70b they read right to left.