I can't comment on the Buckshot, but I would suggest being careful with the Riva carb. I used one a few years ago with good success, but Riva seems to have changed something since then making tuning quite a bit more difficult.
I have ran a Full Spectrum, the Riva Powerbomb, and a regular Mikuni 46 SBN back to back and found that the 46 SBN worked just fine. Very comparable to the other two and much cheaper.