I have owned the 404og, the 404sx the 555 and the 303 before, here’s my few cents on the differences between them, for anyone that is interested in these machines and is wondering what to cop.
The 303 is the classic one, the one that madlib used a bunch etc etc.
Like @reignbear already pointed out, the main hype about the 303 is the vinyl sim comp, which imo lives up to its name, if you level things correctly (drums relatively loud and sample pretty quiet) you get this warm, compressed sucking sound.
MFX12 is quite good too, its a lot more potent than the vinyl sim comp and it also gives you more control with attack and release. Overall the lil box has some really neat and unique effects, I have to mention though that the 505 uses the exact same FX engine as the 303 so that might be an alternative to look out for.
One thing to note is that it is quite painful to make a full beat on it imho, it uses a smart media card which is a lot slower than the CF cards that the 404og/555 uses and the SD card that the SX uses. Only having 8 pads does not help either.
The OG is another really popular machine, unlike the SX the vinyl sim on it does have compression on it, which is actually quite similar to the 303/505. The main difference I would say is that has more high frequencies and a little less noise. The big thing it has over the 303 is DJFX, which is extremely fun to use for live sets, it also has more pads and its a lot faster so its more tolerable to use.
The 555 is my personal favourite, it is amazing to use for live sets, the d-beam filter on it is no gimmick and you can save up to 16 effects and their settings to the effect memory bank, it has essentially the same FX engine as the OG but with even more effects.
Making music on it is fantastic too, its the only SP with loop capture which comes with overdubbing and other features (which you can trigger with a footpedal). Heres a vid of mndsgn showing how he does it:
The mic preamp, the USB midi, and velocity sensitive pads, its a fantastic package. The only downside is that it does not have comp on its vinyl sim, instead its got a filter. However it still has the same MFX12 as the OG.
Then there’s the SX, personally this is the SP that I had the least fun on, it has a completely new FX engine, and you can definitely tell. Making music on it standalone is nice though, it uses an SD card which makes truncating and resampling a lot faster. Personally I generally don’t like the SP workflow so I value the FX and the live capabilities more, but if you are looking into an SP to make standalone beats on then this will probably give you the smoothest experience.
Theres also the 202 which I haven’t owned but I have used, its actually a pretty cool machine, it has some really lofi effects, a lot of distortion and bit crushing, and you can find it for a bargain a lot of the time, though it is a pain to actually make a full track on.
Vulf comp has also been mentioned here a bit and I can also confirm that it is amazing and in a lot of cases I use it over the 303, it offers a lot more control which none of the SPs really have, which is really valuable when you want to mix a track well, goodhertz also makes a ton of other coll plugins, if you haven’t already you should check them out…
ATM I only have my 303 and 555 left, mainly for the live sets that I do, and honestly part of me has been thinking of parting from my 303 because of vulf comp, but the other part of me knows I will probably regret that decision.
I can go into a lot more detail if anyone has any questions.
BTW sorry for the long post 
