I own and love the k5000s. The key action is nice and the build is pretty solid. The PCM section is useful for adding hard transients to the additive sounds, but I don't miss the extra samples from the W version.
This is a synth that requires some serious menu diving unless you have a computer editor (suggested) I use sounddiver on a crusty old os9 mac, but I believe you can find some free editors online. When I bought mine it came with a windows version of sounddiver that was just for the k5k. If you're a windows user, make sure to ask the seller to dig up that disk! It will help save your sanity! The payoff is well worth the menu diving time if you can't find a computer editor though.
One reason I went with the S version was for the knobs. While they only cover a few of the k5k's wealth of parameters, they are actually a big help when you are tweaking your sounds and programming. Four of the knobs are assignable, and the rest give you access to some useful additive and subtractive functions like:
low and high harmonic balance, even and odd harmonic balance, formant filter speed and depth (you can do some really interesting things with these in realtime), you also have the obligatory cutoff and resonance knobs among others.
The knobs on the S don't cover everything, but they can save some menu diving for fast tweaks and they offer some interesting realtime options.
If it isn't already obvious, I would say go for the k5000s