Hello,
A big part of that equation is lenses. Do you have good lenses for either one?
Is stabilization important to you?
Is the D800 weather sealed? Is that important to you?
Is size and weight important to you?
Is autofocus critical for you? I don't have experience with D800 but I've read it is great, but I have no idea how it compares to KP.
Is flash exposure critical? In my very little experience with some friend's Nikons, that is one of the few things I like about it.
Is high ISO important to you? I think the KP may have the upper hand there.
Is frames per second important? The KP is 7 the D800 is 4.
New vs used? Do you know the story behind those 50,000 clicks?
If it was my personal choice, based on these question the KP would win, but what is important to me may not be for you.
Remember that the body will dictate the system: lenses, flashes, accessories, etc.
But here is a tie breaker: This forum!
Best of luck in your decision!
Thanks,