I have a Synth 9 and a B9.
The Synth 9 has a great Oberheim setting which can hold a note in the background whilst you chug a riff, like rock bands use synth players to fill out the sound.
I bought the B9 on impulse; when I plugged it in and started twiddling the knobs I found a Hammond setting and just started playing "Lazy".
These "9" synth pedals contained canned sounds for you to use; more like sampled sounds than a programmable synth.
Programmable synths are something else; I've just sold a MarkBass Super Synth (SS) and it was pretty good.
I've also got a Future Impact, which is much more comprehensive (but is large and also needs a separate MIDI switch pedal to control it). Both are programmable via a PC app.
So, the choice is canned sounds or roll your own.