Its a pretty well known that pentax is still playing [URLhttps://www.youtube.com/watch?v=gPAu0q6rRSE]catch up with regard to aufofocus[/URL]. You wont get any advantage in low light. I guess there is the metering thing but i usually use manual exposure for low /light stuff. My d600 had no trouble metering at night in downtown (there are some pics on my flickr.) During day time, the af will destroy the k3. You have amazing subject tracking (even with with subjects moving towards you which have typically posed enormous challenges.) Go watch the drtv review. The hit rates on their af tracking tests were basically 100%. So, i dont know, a little advantage at night (for something thats not really af) or a huge advantage for the d750 during the day. This is seems like a no brainer. Im also seeing some reports of underexposing.
If you use wi fi too (and i believe this is something that can com in handy for many photographers), the d750 destroys the k3 here too as its all built in and much, much less painless. No need to sacrifice one of your sd card slots. Much better video too on the d750 as well. The k3 doesnt even have continuous af in video. The k3 records at low bit rate and from what ive seen video on the k3 just looks tedious. With the d750 you can get modded firmware that lets your record at a whopping 64 mbps. Part of the upside about nikon is that you get access to a much larger selection of 3rd part things. You will be giving up the full frame advantage. Full frame gives you better sharpness, dynamic range, etc.
I would say that imho nikon has the advantage when it comes to lenses too. Nikon offers a wider selection of lenses. For a little over $200 you can get the nikkor 50mm which fits both full frame and aps c cameras. It has fantastic image quality and near silent af. If you want to get the same thing with pentax.. well there isnt really anything like it. Alot of pentax's lenses tend to be pretty noisy which poses problems for video.
Bottom, line is that the d750 is all around very solid package for stills and video. There are lots of things that can be expressed in a specs sheet. I love the sleek, modern look of nikon cameras, how easy everything is to do in the menus. Shooting in live view is almost zen like. I didnt even bother cracking the menu when i got my d600 and i was still up and away shooting.
Plus, you have to consider that you are trading a $2000 camera for an $800 one.