Well I ended up with both the SYB-5 *and* the BSY-600. A bandmate gave me the syb and the expression pedal to go with it, and I got an amazing deal on the BSY on eBay - $28!! I still need a second expression pedal for the BSY.
I'm using them both in my rig. I did notice severe tone suck with the Behringer; minor but noticeable with the boss. I got a One Control "iguana tail loop" true-bypass switcher (5 loops) to run them through which takes care of that. (I settled on this brand due to the quality, price, dedicated tuner output, and a 9v output for each loop which creates several options for daisy chaining your power through your pedalboard.)
They both make usable sounds. They have nearly identical controls - the Behringer may actually be easier as the direct/effect knobs are separate, while they're combined (inner/outer knob rings) on the boss. Based on a small amount of messing with them, I'd say the boss makes better synth sounds, while the Behringer sounds more musical to my ears. I have the boss set to a fuzzy sound that I can weave in with the pedal, and the Behringer to a funky envelopey filter sound.
The "hold" function that both pedals have feels useless to me since you have to keep your foot on the pedal to hold it, so you can't do much else while it's on. The footswitch is quite flimsy on the Behringer, as expected. The expression pedal jack on the boss is in an annoying place that gets in the way of the audio cable. I forget if it's in the same place on the Behringer.
I looked at the boss syb-3, but I would certainly only recommend the SYB-5 over the 3, as the 3 doesn't have the expression pedal control.
If you don't want to invest in a bypass looper, the boss is better than the Behringer. But with the addition of the loop, they both work well, just slightly differently.