Originally posted by Mattox I really feel that what the D810 and 750 offer are far more superior than what Pentax has at the moment
But did you try things yourself? I don't know what camera you have, but you should test it against a) the best that Pentax is offering, and b) the camera you are thinking about switching to. You can try renting the camera or finding a local person that has it, so you can see it in hands. Just because you see some great photos taken with Nikon gear doesn't mean its just the gear - its the photographer that knows how to use it. I've seen some really great sports photos taken with manual focus, for example.
Originally posted by Mattox and their range of FF lenses also
True, Canon and Nikon are producing more lenses than Pentax at the moment. And they have a little more third party support. Their legacy glass adaptability is generally not as good as Pentax'. But do you really need those extra lenses? I mean, its one thing if you need, for example, a 400mm f2.8 lens, something that Pentax doesn't have. But its another thing to just fantasize about the many, many lenses, even though you will never actually need or buy 98% of them. No point to switch to Nikon just to buy their cheapest 50mm, a kit zoom, and another lens that you already had in K-mount, you know?
I'd suggest you try to define exactly what is limiting you with your Pentax, in very specific terms. Which lens, specifically, do you need, that Pentax has no alternative for. And which AF mode, in which cases is not working well enough for you.
We had a couple of threads of people who also fantasized about other brands and moved to Nikon and Canon, particularly when those two brands introduced the "affordable" FF cameras (D600 and 6D?), and some of them expressed regret. Others started using both brands.
Its for you to decide, and Nikon certainly has some great gear out there. But a move is costly, especially if it turns out to be a mistake. Make sure you know exactly what you are doing, and why.
Oh, and I would definitely wait until the official FF specs come out. I expect they will update the lens roadmap around that time, as well. That could make current Pentax cameras more affordable, and inform you of what to expect with Pentax in the future. Maybe Nikon will lower some of their prices as a response, as well.