I am trying to really figure out what the difference between these is going to be in actual use. I realize the 555 is a bit bigger and has compact flash and more effects, sx has sd card storage and 404 uses compact flash, and both of the 404's can be portable which could be nice.

Has anyone tried all or 2 out of 3 of these. Which one do you prefer?

I'd like to set up a small rig with one of the sp's and my Zoom HD8CD recorder....would like to sample from field recordings, electric and acoustic guitar, monomachine, various percussion, portable Numark PT01 turntable.

SP-555's main points of difference IMO:

- the live loop track for improvising a little bit
- Velocity sensitive pads..!! Very great
- audio interface for computer

404SX:

- SD/SDHC card.. bigger, faster
- Sound quality.. better (the others still sound fine tho)
- Slightly different FX
- Swing

404

- Cheapest
- Better vinyl sim than SX (apparently)

Really I would say this:

If you don't need portability and can afford it, get the 555.
If you need portability and can afford it, get the 404sx.

If you're poor/need portability - 404.

well put cartesia