I have the K10D and I've used my GFs sister's GX-10, although I am used to using the Pentax system I preferred the K10Ds menus than the GX-10... Having said that, what comes out of the camera matters more and they are both equal in that respect....

The Samsung SA lenses and the Pentax DA lenses are good, the 50-200 I believe is better than the Sigma, depending if you get a good lens or not.

Check that you can still get a rebate on the GX-10, I think you can in the UK but I am not sure in Italy, it's worth checking, just to save some lire (oops Euros).