All I can add is that I came from being a die hard Canon shooter and L Glass to Pentax back with the K10D. Leaps and bounds above the Canon junk! Most Canon shooters will NOT give you the real info in there! Canon has, and has been having, non-stop QC issues that have been progressively getting worse and worse! I got so sick and tired of the BS it is what drove me to buying Sigma, Tamron, and Tokina lenses instead of all L glass, eventually drove me right out of Cannon products completely and into Pentax. Many of my close buds shoot Canon still, those that haven't jumped ship to either Pentax or Nikon already, and many of those currently shooting are ready to jump ship now after the 1Ds & 1D MkIII list of fiascoes and faults and constant issues. All the issues even with the 40/50D cameras as well, focusing issues, shutter malfunctions, etc.
My 20D hand banding issues(like everyone else, mess up with camera), promised by Canon a new firmware to fix the problem, never came/happened! Had to send Battery Grips back due to causing shorts, fixed that, out BG for 6wks+. Faulty flash mechanisms on the 20Ds, and the list can go on.
While Pentax has some quirks here and there to learn/get used to, I have had NOT a single issue with any more my Pentax gear to date! Even far better IQ then my Canons + L glass ever put out!
Don;t get mne wrong, I loved my Canon rigs, just got tired of all the BS with Canon not giving two craps about its clients! And instead of fixing bugs, issues, etc. they just would release another model in the same line fixing those issues(supposedly) and expect you to have to purchase an entirely new body!!! Add a few extra useless features on there to make it look better, etc. Total BS! L glass with tons of AF issues, IS units breaking left and right on 70-200mm f/2.8 IS USM L and 100-400mm f/4 IS L and 300mm f/4 IS USM L lenses, internal components falling apart and into the lens barrel on an entire batch of 100-400mm IS USM L lenses, faulty shutters on Rebels, 40Ds, etc. Giving Spot Metering that is NOT even true spot metering just something they are calling spot, I mean, come on, even the entry level from all other Manufacturers tend to offer true Spot Metering as it is an integral part of photography! Now the fiascoes with the Series 1 bodies that have been plagued and junk since day 1!! They need to stop trying to live off their name and actually start producing some great equipment and lenses like they used to years ago!!
Currently the only camera body Canon makes that is worth a poo is the 5D MkII, and even it has its issues, sadly. I almost bought one to have a 2nd system to go with my K10D(soon to be K7) and film cameras. But just couldn't justify being pretty much stuck with paying premium, crazy over priced, fees for Canon's lenses to use on it, not with all the current QC issues especially. So, I ended up snagging a brand new Sigma SD14 for under $400. Far better colors(true colors) over anything out there currently(Canon, Nikon, even Pentax, only Fuji with their SuperCCD comes close), uprezed files easily exceed the quality of the original Canon 5D, so god to go there, DR is insane to say the least, reminds me of a MF digital, almost, seriously! The depth is just simply amazing. And I cann pop out the Ir Cut pass filter manually and now I have a full time IR & IR+VIS capable camera as well!
K10D does well enough for me for everything I shoot including the occasional action shots. But I also came from shooting sports/action with older full manual SLRs + MF lenses and still got the shots, so I don;t have to rely on speed of the camera as many newer shooters do. Now getting older and slower though, I do like that the K7 has a nice faster FPS, to me 5FPS is perfect for whatever one could ever need.
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But, if you have to choose a different system over Pentax, look into Nikon, NOT Canon! Biggest issue with Nikon is lack of VR on most lenses.
. Sigma is however slowly adding OS to more and more lenses as well as almost all lenses are now coming with HSM as well. They even just recently released a 24-70mm f/2.8-4 HSM OS lens! If they just add OS to the 70-200mm f/2.8 or the 100-300mm f/4 or the 120-300mm f/2.8 as those lenses rock big time!!!!! The 120-300mm and its little brother 100-300mm even work great with a 1.4x TC on them!!! 120-300mm even works well with a 2x TC on it, and will produce acceptable images with a stacked 1.4x TC + 2x TC on it! The 100-300mm will do acceptable images with a 2x but dies with them stacked, sadly. What rocks though is the Sigmonster 300-800mm f/5.6, lol. *DROOL*
I saw you mention the Canon EF 300mm f/4 IS USM L, that is one excellent lens, big time! AF/IS is a bit loud though! But IQ is amazing! Add on extension tubes and it even worked well for long distance macros too! I loved mine wen I had it. But the 100-30mm and 120-300mm both had about the same IQ once the images were in print. Same goes for the 120-300mm(even with a TC) produces about same print quality as the EF 400mm f/5.6 L lens does. The 100-300mm f/4 Sigma will produce about same quality in print as the 400mm EF L, but not with the TC attached it will not, a tad bit less, but only noticeable at larger sizes, 16x20" and up.