You should definitely try to get a good exposure first. Both of those shots are underexposed. Bringing this exposure up to "normal" (we don't know what software you are using, but some do this automatically) will increase noise a great deal. Couple that with the fact that the Pentax shows a -1 exposure bias means surely your are under exposing. Don't make any judgements without using the camera properly.
As others have asked, are you shooting RAW or jpeg? If RAW, you must apply your own noise reduction in your software. If jpeg, what is your NR set to?
A properly exposed shot will look a lot better than an underexposed shot.
Here's a K-5 shot at ISO 5000 -- so this camera can certainly deliver better than you are doing so far.