The K10D will be a camera you are less likely to outgrow and doesn't have some of the rather annoying limitations of the K100D (no SDM lens support, no wireless flash, no front e-wheel, tiny 3-frame buffer, missing some useful physical controls, smallish viewfinder, some of them are finnicky with battery life) - I think no matter what kind of photography you are into, you will eventually find yourself missing some features of the K10. I'd say get a K10D even if it means spending a bit more.
The K100D was my first SLR and got more than 3 years of solid use - then again, I spent a large portion of these 3 years kicking myself over not getting a K10
Also, I recall a deal where you could get a brand new K100D for £160 - quite a bit less than $300, and UK prices are usually a rip-off - you should be able to do better than that.