| Just to add something to that very useful thread, due to a recent bout of insanity i currently have both an xp-100 and a whammy-4 on my board, there are a few differences especially on bass, like that thread said, the wh-4 tracks slightly better on the low notes, but to be honest i never use it that low so it's not an issue. The tracking is slightly different, the xp-100 is a little more watery, less focused, but fuller (if that makes any sense) than the wh-4. The xp-100 also has more modes if you didn't know, it has wah's as well, but they''re pretty much unusable on bass, except for the envelope filter patch which i use all the time set heel down for a perfect boomy thump.
The xp-100 also has the 5'th up/octave up harmony that the bass-whammy has but the wh-4 doesn't, which justin chancellor of tool uses for the song disposition, which is a big selling point for some people.
I do prefer the construction of the wh-4 though, it feels a lot more solid, the expression pedal on the xp-100 feels a little flismy, but it's held up so far, also on the xp-100 you can adjust the modes with a footswitch, and it has user presets, on the wh-4 you have to turn the knob to change modes.
Eventually I decided to keep the xp-100 because I couldn't live without the envelope filter patch and the sound is more suited to my noise/experimental sound, but if i could afford it i'd keep both | 
