I have had both pedals on my board. IMO, this is an easy choice of the MXR M82 bass envelope filter. I could get infinitely more use out of the envelope filter than the boss SYB5. I don't know what your style is so this could be just my opinion.
the Boss SYB5 was a bit useless for me. the tracking is kind of tricky. you cannot play any hammer-ons/pull-offs, you have to play the notes one at a time and very clearly otherwise it'll glitch out. perhaps you want that though. it has a few interesting settings but it definitely takes a while to get some good settings I could use and then once I did there really isn't any application for which I would normally need this pedal. I play in a cover band every weekend in redneck biker bars so it's mostly classic & modern rock, country, and blues.
i have had the MXR M82 bass envelope filter on my board for about a year. it is super funky and it took me absolutely no time at all to find some settings that I liked and were musical. I could use this pedal for a lot of different things and in combination with some other pedals like fuzz, octave, and chorus, I can get some pretty useful synth tones.
get the MXR M82, end of story.