I like how you named the pros of each system,
in order of importance.
It looks like you're doing the research, which is good
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I can't comment on the K20D, as mine won't be here until Wednesday. I can't comment on the 5D, either.
Looking at your list of "importances", though, it really looks like the Pentax list simply trumps the Canon list, offering more of a "bang".
For example:
Weathersealing - You state the conditions of use for the camera, and the fact you've lost cameras before. An important point! A camera is not good if it's broken.
vs
Image quality -- Both cameras, I'm sure when paired with appropriate optics, are outstanding. Would you be able to tell the difference between a 5D and K20D shot? (I'm curious - I have never studied med. format shots).
Is this difference (whether perceived or real)
greater in importance than weathersealing?
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Price -- This is important to me, as I'm not terribly rich. THe K20D and system can be had for pennies compared to the Canon. This may not be as large of a concern for you compared to ...
vs.
Full Frame Future -- A toughy, as I've never been technically inclinded enough to really learn about the intricate physics behind cameras. Again, though, I think "practice" vs "theory" comes into play - Yes, diffraction limits may be different - but would you
really see the difference on screen when not 100% zoomed? On print? I don't know, but important question to ask.
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I think only you can decide which of these factors are more important to you...every photog is different, and wants different things from the camera system in use.
I wish there was a perfect system, that's for sure, but until then we have to make compromises. Some choices are very tough for some people, while some choices are easy because of a huge limiting factor (money) makes one brand a sure win over another.
I personally chose the K200D after lots and lots of reading of reviews of the body compared to similar Nikon and Canon systems. The huge win for me, I think, was the sealing and the in-body stabilization and (older) lens selection.
Lucky for me, I am not a huge sports fan or photographer, otherwise I would be sorely disappointed, and I don't need (nor can I afford) huge telephoto lenses. For me, at least, Pentax matched my most prized criteria, while weak in the ones I deemed not as important.
I think you too should decided which of the points you present are
more important and go from there -- There is no perfect system.