I'm building another X99 system, I want at least 8 cores, gaming performance is high priority, and I will be watercooling+overclocking (EK-CoolStream XE 480 for the CPU).
Simple question; 6900K or 6950X? Money is not a consideration.
If you're gaming, the extra cores aren't going to help, but the higher OC potential of the 6900k will. You might even go with the 6 core version to get the higher clocks.
Yeah the 6950x is about $170 cheaper ($1580 on pcpartpicker) than it was back at its launch date, making it a slightly better deal, but it's a terrible overclocker.
If you have a 5960x already, it's either get the 6950x or keep the 5960X. The 6900K is not worth it at all (it clocks similarly and there is a very small IPC increase). Only get the 6950x if you really need the 2 extra cores, because you won't hit the higher clocks that you can get with the 5960x. Then again, if you have a program that can make use of many cores, it might be better to get a Xeon instead.
I wouldn't call it a lame duck. I just use a Corsair h115i with some Noctua NF-A14's on it for my 6950x and I've gotten it up to 4.4 at 1.38v with temps around 80c under stress. For every day use though I keep it at 4.3 with no issues, temps top out at 74c under stress (25-45c average during 4k gaming/streaming). I'd love to try and get more out of it but I'll have to build a custom loop for that...