Good Morning and Welcome to the Forum,
I have not shot with a Nikon for 40 years now (since back in Navy), so I really can't add a personal real world comparison. Having said that, if you do choose Pentax, it will be different - some experiences for the better and some for the worse. A mixed bag at best. A lot of the comparison points will be based around your specific shooting style - so they will be different than that of specifically cited tests.
- On the topic of buffers - they only exist in order to mediate the rate of images from the camera (fast) to the rate that the storage device can store the images (slow). Its first in - first out (FIFO) and is fully dependent on the rate and timing (burst rate) of you the photographer (i.e., style) and the speed of the storage device (the SD Card - so a fast SD Card(s) will make a large difference). The images will start to be stored immediately, and the camera will only slow down when there is no (adequate) buffer storage available. Image capturing will resume as soon as buffer storage becomes available. So your specific style and burst rate will be an important factor here. Using the absolutely fastest SD Card will also be a large contributing factor. The K3 also uses USB v3 as the transport mechanism, so the speed of the buffer and its interface to the SD card should not be a factor. YMMV - fast image production with slow storage media = problems. Also with the K3 two card slots, you will need matched cards (in terms of storage and transfer rate) to ensure that the buffer stays as empty as possible (e.g., you really do not want to run with one fast card and one slow card - for best results).
- AutoFocus - Nikon is one of the fastest AF bodies available (not wanting to get in the Canon / Nikon argument here). This goes with out saying that both C & N are used for sport shooting at the professional level. The Nikon D7000 is the equivalent of the K5 (both use the same sensor). Pentax then improved it (AF and AF in darkness) with the K5II/K5IIs models. Further improvements then went into the K3. So, that is a bit of history.
AF comparison is at best a bit muffled between camera brands. Each brand has its own system - specifically it own algorithm, which emphasizes or de-emphasizes certain aspects of the overall problem. There is a German photography magazine that did an extensive study in the autofocus designs of each camera vendor. They came up with some surprising conclusions. There are two types of AF designs - one for speed and one for accuracy. For sports you use speed - hence Nikon, and for accuracy - hit rate for having the image in focus - Pentax. The magazine actually went out and had engineers design the experiment in order to be fair and accurate to all camera vendors. There is a pretty extensive discussion about the results in this link.
One of the factors in AF is the lens, and the magazine tested across a large number of lenses for each vendor. A good AF lens has a very short focus throw (the range of motion across the focus) versus manual focusing lenses where you want the largest amount of focus throw so as to be able to fine focus the object. So, what you really want is a AF lens with a very short focus throw. There is also some menu based AF adjustments that will probably help - specifically the "AF Fine Adjustment", item 26 in the C menu - 4th page.
There is also (for Pentax) the AF implementation method. Pentax supports both the old mechanical screwdrive as well as the newer electric focus motor (SDM and CF). This will have an effect on your AF speeds in particular (in terms of implementation), although the algorithm will emphasis accuracy.
Lenses in general with Pentax is going to be a problem, even with third party. Nikon will have a much larger selection - especially when third party is included. Its just one of the things you live with in Pentaxland.
So, even on something like AF - its not a cut and dried comparison - the devil is in the details.
Last edited by interested_observer; 08-10-2014 at 11:05 AM.