This thread has been silent for a month so I wanted to at least give an update, if not put a period on things.
Simply put, I will not be switching brands, at least not in the near future. No earth shattering reasons. The decision was equal parts logic and emotion. And a little bit of timing.
On the emotional side, I know and love the stuff I shoot with, and had an opportunity to consider switching not been laid at my feet the thought would have never popped in my head. Plain and simple, I like what I shoot with and I like the fact that I chose to shoot with it.
On the logical side, I'm a high end pro-sumer shooter, and the stuff I was loaned (permanently or otherwise) was pro gear. Is it better than what I have? Uh huh. Not in every way, but in very important ways, including focus accuracy and speed. Do I need that? I can tell myself that I do, but my livelihood as a photographer doesn't depend on these things, so I can tell myself I might want it, but need is secondary. On top of that, comparisons to my gear are apples and oranges. If I had a D4s and a D810 in my hand, and a set of f2.8's instead of f4's, then I could judge whether the differences I could see were significant or almost nonexistent. So, if I were to change the question then becomes, "Are you trading up to 1d's and 5d's, or are you looking at a 6d to replace the D750, in which case what are you really gaining?" The truth is, I don't want the pro stuff, nice as it is. If the D5 doesn't make me want to trade up then the new 1d won't either. Plus, even if I'm getting a discount on new gear I still have to sell my old stuff off at a loss, and D610's and D7100's aren't bringing in lots of $$ these days. So at best I'm in a situation where I have some great glass to borrow while I slowly sell of stuff to replace it with the same stuff and slowly build inventory again. Not a bad thing, but only if I want to switch.
On the timing end? I was already where I am with the decision as laid out above before the announcement of the D500. The announcement was the death nail. Not that a 7d Mk III won't be out in a year, but with the $$ I'll save by not having to sell off and trade I can certainly look into some glass that will make my upgrade to the D500 even smarter down the road.
What this doesn't say is that I think Nikon is in any way better. What I've learned is that they're just different. For everything I'd gain I'd lose something I'd miss. The truth is that if I ever decide to ratchet up the rig then, should circumstances remain the same, I'll likely jump. Until then, I'm happy not to have to retrain myself.
So stick a fork in this thread.
PS - Playing with a 300mm f2.8 is not something I'd recommend unless you're ready to jump on one yourself. That's gonna be tough to say goodbye to.