Between your two choices, and given your past experience with DSLRs, I do think you'd benefit from upgrading to a more professional camera. Choose either the Pentax K-30 or the Pentax K-5. Now to answer your questions:
First off, given the choice, go for a Pentax over ANY Sony DSLR. Sony has some horrible history going for it, and their latest DSLRs use a strange translucent mirror which actually causes image quality to suffer. They also use an Electronic View Finder, which is less detailed and requires the camera to be on in order to work. A Pentaprism is the best viewfinder, followed closely by the pentamirror and rangefinder OVFs.
Next up, Pentax cameras are compatible with ALL Pentax lenses. Each Pentax lens ever produced has a servo drive coupling at its base, allowing AF operation with all Pentax bodies. The Sony lenses of the past, meanwhile, will not Auto Focus with certain bodies and there is no guarantee that they even HAVE the servo coupling. So Pentax wins at its lenses as well.
Finally, low resolution does NOT yield sharper images. There is a measurement that can be taken called an "Airy Disc Dispersion Ratio" which simply means how much points of light the actual image will be created with. On most high-end DSLR-like cameras (I'm assuming you mean Bridge cameras) this ratio is around 0.2x meaning that it takes up to FIVE PIXELS to record a single point of light. On the Pentax K-R and K-30, this ratio is a solid 1.0x meaning that each pixel records one point of light. This yields extremely high resolution images which are tack-sharp and blur-free.
With its translucent mirror technology, newer Sony DSLRs max out at about 0.8x ratio.
Hope this helps, I own a Pentax line-up myself and the results are STUNNING. I do own two K-30s (used to have two K-7s) and one Pentax 645D as my workhorse camera and I can guarantee that going with Pentax is the BEST possible option for anyone, better than any Canikon.
- Mike, SvitekPhotography and DPRChannel
