Well, I can only comment on the BOSS SP-303 and Yamaha, since I've worked on both.
I picked the BOSS SP-303 over the Yahama. Why? Well, more effects, you can apply the effects at 44.1Khz, better polyphony (multiple sounds at once), and it was $40 cheaper. I payed $299 for my SP-303 at Guitar Center. Also, on the 303, you can have both line-in and a mic connected, and they will both work (can turn mic down/off with control knob, or just turn it off with the switch). On the SU200, the mic cuts out the line-in when connected (even if switched off). My only (minor) problems with the SP-303: When importing .WAV files from the computer via SmartMedia, you have to give it certian file names (not that big of a deal, really), and, it uses RCA plugs (not 1/4"), and no battery support (again, not that big of a deal, but would have been nice to carry around and sample with).
The Yamaha SU200 looks nice on paper, but in reality, it sucked compared to the SP-303. The 'scratch' ribbon wasn't that cool, it's effects were limited, time stretch introduced distortion. You also CAN NOT apply effects at 44.1Khz sampling! And, if you do use 44.1Khz sampling, you can only play 2 pads (samples) at once. The ONLY good thing I'll give the Yamaha is it's 1/4" phono plugs. Oh, and when you import .WAV's via smart media from your computer, you can name it whatever you want. But, that's it.
For the Zoom, check out
http://www.harmony-central.com/Synth/Data/ZOOM/SampleTrak-224-01.html If you can find a store that has all 3, I'd say check them out personally first. I just use my SP-303 to trigger samples, I don't use any of the 'sequencing' functions. I don't know if that's important to you or not.
Later,
Bill