I'm a beat maker (not an aspiring music producer, just someone who likes to push buttons and make noise), and I won't be using it for much DJing purposes. I am currently running Reason and Recycle with an MPD24, so velocity sensitivity isn't a must. Whichever one I get is going to be chopping samples a lot though, so I'd like it to have a good workflow.
The things I like about the SP-404 are that it's portable, it seems to be reliable for most people, and it's a little bit cheaper (money isn't TOO MUCH of an object
What I like about the SP-555 is the 16 pads instead of 12, USB connectivity and the fact that I wouldn't need an interface, and the extra features and effects (although that D-Beam or whatever it is doesn't interest me much honestly.) The problem with this one is that I've heard it's a little bit glitchier (being newer and everything), which is a big red flag to me.
So that's about it, if you'd like to know anything else, please ask. Thanks guys, any tips will go a long distance for me.