I own both the original K-5 and have owned the K-3 for over a year. My passion is the QUALITY of every shot I take. This is where the lack of AA fuzziness on the K-3 and its larger photos make a helluva difference. I've never had to use the anti-aliasing option. Every lens you own will get sharper results with the K-3 over the K-5. You can also do more with crops on the larger K-3 files, giving you much more latitude in composition. With good crops, it's like you automatically get more reach over similarly composed shots with the K-5. I also find that autofocus is faster and more precise with the K-3. There's a small learning curve on the controls, but after a month or so, your muscle memory will kick in and the differences are irrelevant. If you pixel-peep, the K-3 seems to top out faster on the ISO scale - I don't like to go higher than 1600 for best results. The irony is, if the photos are re-sized to K-5 dimensions, the ISO results are identical. You can't tell the two apart as far as noise is concerned. My "green" setting allows auto ISO selection to 3200, and the results are acceptable, in reality. It's just that I'm really picky as a semi-pro photographer.
The bottom line is that I use the K-3 pretty much exclusively, and keep the acceptably satisfactory K-5 as a backup.
At the same time, I'm looking forward to the introduction of the announced full-frame Pentax DSLR and professional reviews. Then I'll make the decision if it will be worth the expense to make the move to that format. I presently have only one FF-compatible lens (the SMC-FA 50mm 1.4). I have a lot invested in APSC - only lenses, and being limited to using them in that format on a FF seems pointless. It will take some great leaps forward in a new FF to convince me to make the switch from the great K-3.
John
PENTAX : PENTAX Photo Gallery artist page