Thanks for cross-posting your question (and thanks for joining the Flickr group!). I've answered you there with my opinion, and I'll cross post it here:
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Thanks for joining the group and posting your question. You are far ahead of me in the practice of astrophotography, so I feel more than a little inadequate in "answering" your question.
I think that there is no question that all other camera brands take a backseat to Canon when it comes to astro-oriented software (thinking especially of BackyardEOS here) and I would love it if Pentax would facilitate a developer making something like that for Pentax DSLRs.
On the other hand, I think that Canon sensors are actually far worse than than the Sony sensors for astrophotography when it comes to noise. The sensor in the K-5/K-5ii/K-5iis is just superb for astrophotography and that same sensor is used in many high end "CCD" (actually CMOS cameras) for astrophotography. Still, one can't deny that, thank to post-processing, stacking, etc. lots of people turn out lots of great astrophotography with Canon cameras.
I think if you are honest, you will admit that your main limitation is not having the jack for an external intervalometer. (Not the camera or the software). That is because the K-r (like its predecessor the also excellent K-x) was considered a Pentax "entry level" camera and not a flagship model with additional functionality like front & back wheels and shutter jack. Secondarily, you would probably benefit from the bigger buffer in a flagship model. Neither of those issues is due to the brand of camera you shoot, just the level of the camera you shoot.
If you were looking for the most "bang for the buck" I would highly recommend selling the K-r for what you can get and upgrading to a K-5. If you could spend just a bit more, the K-5iis would be even more beneficial for astrophotography for its lack of an AA filter (something you can't get away from in a Canon without going to a Da or an expensive (and risky) modification. Then you could use a cheapie intervalometer on the best 16MP sensor out there (14-bit camera), since they all have jacks.
The advantage of that sensor is that, due to its low noise and high ISO performance, you can get results equal to the Canon with less integrated exposure time (meaning better use of your time in the dark). To me, astrophotography starts with the ability to capture as many photons as possible and that has to do with the sensor and the imaging engine in the camera. If you look at the stats for the sensor, comparing the T3i and the K-5 (and K-5iis) you will see that you are giving up A LOT with the Canon sensor:
DXOmark Comparison
Specifically, you are giving up 2.6 STOPS of dynamic range and you get 50% MORE high ISO performance. (In film days that would have been like being able to shoot at ISO 1200 rather than ISO 800, all other things being EQUAL).
There are lots of (often low mileage) K-5 to K-5iis bodies available on the used market with people desiring to purchase the K-3. You can pick one up for an amount similar to what you would spend on a new T3i body.
I can't tell you what to do, but I think you owe it to yourself to at least try a K-5 (former flagship) family body before abandoning it for a Canon entry level body. Economically, you would be able to get nearly all of your money back if you decided to turn around and sell the K-5 (since buying used you have avoided all the depreciation of a new camera, the minute you walk out of the camera store (or open the box). Hope this gives you some food for thought.
