I've used both. The MK5s are a bit brighter than the MK4s, which I like. (Then again, the MK4Vs are also brighter than the MK4s, and I don't like them. Whether it's because of the more abrupt bump, or because they're less directional at higher frequencies, I'm not sure.) I really only think the MK5s are worth the extra cash if you also want and will use the omnis. I think both the MK4 and MK5 sound great, but my ears give an edge to the MK5s. I've only used the omnis sparingly, but really like them when in the right situation - usually on-stage or unamplified performances - either with a small split or better yet a j-disc.
At any rate, I think all three - MK4, MK5, MK4V - are suitable for a variety of recording situations. Just depends on your preference, whether you want the omni option, etc.