i have the sp-555. i am thinking about buying a 404 mainly for the portability. (theres lots of times id like to jam when im away from home.)
anyway i started on the 555 so what would i miss from it that the 404 doesnt have? now im not talking about the loop capture, usb, or phantom power. im talkin more like the functions when your playing with the samples or constructing patterns. THanks.

you will miss the lack of dedicated sample and pattern copy mainly. Also the pads are not as responsive (some people prefer them though) and the inability to sync effect tempo to the pattern instead of the sample. You will gain battery power as you mentioned and the infamous vinyl sim compressor.

probably obvious, but you also lose velocity sensitivity

unless you use a midi controller

No looper so no pattern resampling.