Owned SP555, SP505 (sold), SP404sx, Microsampler (sold).

555 (WINNER)
Pro:
-loop capture
-Great effects, more than 404
-Nice pads
-Fast to use
-D-beam for filter
-USB interface for recording or sampling into box
-Resample patterns using loop cap

Con:
-(common SP stuff) no good pitch, no good timestretch
-Backups only contain bank A and B
-Now they are expensive

404sx
Pro:
-Fastest of the SPs, faster than 555
-No compression (wav)
-Great effects
-Portable, batteries, decent pads
-Backups contain ALL samples

Con:
-No velocity
-Less effects than 555
-Can't resample ext. source
-Can't resample patterns
-(for some) very clean sounding , no "character" like 202,303


505
Pro:
-Auto chop
-Vinyl compression effect (ducking)
-Grittier sound
-Nice screen
-4 Track sequencer

Con:
-Other effects dont match 404/555; I ended up using only ducking comp (vinyl)
-Slow, slow, slow smartmedia. Load, delete, chop everything adds another span of seconds that added up
-Pads weren't my favorite