Many people (especially those who have never used both cameras) mis-believe that they are close and similar enough and that the IQ *should be* the same. Yes, on paper it probably is. But after prolonged and extensive use of both cameras myself. I do find them to be quite different in many ways, from the IQ, to operation, and accuracy and them system performance, which I list the obvious ones as below:-
IQ aspect - colour response
The colour response of the K-x and K-r are different. With the same Custom Image (CI) and the same default parameters, they produce different colours. This can be confirmed when using the same lens and take pictures on the same scene side-by-side. Just say the Natural CI of the K-x is less saturated than the Natural CI of the K-r and that the Portrait CI of the K-x is more colour accurate than the Portrait CI of the K-r, which produces exaggerated Canikon entry level DSLRs alike colours.
IQ aspect - noise handling
The noise handling and behaviours at higher ISO of the K-x and K-r are somehow different. It seems that the K-r processes noise in a smoother way and image is less "discrete" after processing, which could look more pleasing especially when viewed at pixel level. However, for some cases, the K-x would retain more image details.
Operation - Handling and Build Quality
The K-x always feel more solid when held in hands, plus it is indeed somehow smaller (and slightly lighter too).
Operation/Performance - LiveView/Movie CDAF
Obviously, the CDAF of K-r is faster. And it seems to be more accurate too. The zooming-in of the part of image being focusing is also the unique characteristic of the K-r (ditto for the K-5), which cannot be disabled anyway.
Operation - Movie Recording
Despite both K-x and K-r are recording movie in Motion JPEG in 720p, I found that the Codecs used in the two cameras are NOT the same! I did encounter different playback behaviours/software compatibility issue with the K-r and K-x recorded videos. Say for example, with the same software player on the same machine, the K-x video could be played flawlessly whilst the K-r's could be played without sound. So, Pentax must have made some changes to the video codec, but yet which we do not know.
System Performance - Continuous AF
The K-r is the first Pentax DSLR that I found that the C-AF does work better and is practical to use in conjunction with the continuous shooting mode. It could keep track with the object with reasonably higher successful rate with acceptable focus (less OOF and blur) than the K-x and any other previous Pentax DSLRs. Indeed, putting a set of Lithium battery into the K-x could help it to focus faster but as for the C-AF successful rate, the difference is not noticeable (even the AF motor does turn faster).
System Performance - Continuous Shooting
Obviously, the actual frame rate of the K-r is much higher than that of the K-x, and is indeed is even better than that of the K-7.
But one thing I note that is in continuous shooting mode, the exposure of pictures in JPEG mode is not adjusted anymore, versus of what the camera will do in single-frame advance mode. As a result, it is quite likely that the user would see more under or over exposed pictures than normal, i.e., in single-frame advance mode. After all, the image processor must adjust something, so as to compensate for the exposure errors with different lenses (which was a true headache with all those older Pentax DSLRs, since the *ist D).
System Reliability - AF Accuracy
K-x does NOT front focus and back focus as much as the K-r when different lenses are mounted and especially under tungsten light environment, which with the K-r large degree of front focusing *must* be resulted. After all, the focusing accuracy of the K-x is better, and IMHO that it is even more stable than the K-5 for what I tested (that was one of the main reasons why I cancelled my K-5 purchase order! :-(). The K-x does have shift to front focus under tungsten light, but that amount of the shift is less than the K-r's such that the impact is not as dominant.
So, after all, I found that the two cameras are different and they do have different pros and cons of each own. But coincidently, they complement each other for many of the pros and cons and as such, I would select either K-x or K-r of mine for different shooting purposes for what each of them will perform better! For instance, if I need better image colour and focusing accuracy, I use the K-x. On the other hand, if I need need to shoot fast object and use continuous AF/higher frame rate, I use the K-r.
N.B. In order to have higher AF accuracy of the K-x, it *must* be debugged for the AF at the first place.
JUST Believe any or all of the above or not, as I would provide no further proof nor evidence. Nevertheless, I do hope that the above could be useful for all those Pentax users who are still deciding between the K-x and K-r, putting aside the most annoying battery issue of the K-x, of which finding a stable rechargeable power source is simply a headache! But then recently I have almost tackled this problem completely, nonetheless. Why I say that it is "almost"? It is just because the total energy capacity of the LiFePO4 rechargeable is still lower than desired, although it is not really much lower than any NiMH before the camera cuts off, just at slightly dropped lower voltage of the batteries.