I've owned both. I just sold my K-r because I purchased a K5 on CyberMonday. I kept the K-x as my backup for two main reasons: it had more shutter actuations and hence would bring fewer $'s to offset the cost of the K5 and it uses AA batteries without an adapter which means one less charger system to worry about.
The higher resolution of the K-r screen and the illuminated focus points were big pluses for me with the K-r. However, I have all that with the K5 plus a bunch of other things.
Some things not often mentioned between the two:
- The K-r has a better pop-up flash - its physically larger and pops a bit higher thereby adding reach particularly with hooded large diameter lenses.
- Focus was a bit faster on my K-r for most of my lenses from what I could tell.
- Genuinely no noticeable difference in continuous shooting when I was shooting my son's soccer with the DAL 55-300.
If you are buying used you have the opportunity to ask if the user has experienced any FF issues with the K-r. I honestly had not, but most of my photographs with it (by far the majority) were outdoors.
I am a huge fan of AA batteries. They just make sense as long as the weight issue does not become a real factor. For the K-X and K-r weight is NOT a factor IMHO. If you decide on a K-r I strongly advise you to get the AA battery adapter for it. You can ALWAYS find AA batteries but charging a proprietary battery is not always in the cards in my experience.
Good luck with your decision.