Hi Robert
I've not used a K-x myself but they seem to be getting pretty good reviews. It really depends on which model you prefer (which I know sounds a bit wishy washy!).
With the Nikon cameras you'd have the advantge of more new lenses as Nikon are constantly churning out the newest lenses. The Pentax should be backwards compatible with older K-AF mount lenses so it's possible your 100mm lens will work on the K-x but you'd need to check it out more thoroughly.
With any luck a Pentax expert will be along any minute now...
