Just skimmed the thread, but I want to drop a warning in case someone else is looking for the same thing.
gt860m can actually be three cards, one of which is completely different. Also, gt960m comes in two variants.
The 960m comes in 2gb and 4gb variants, and that's the only difference. Get the 4gb if possible.
The 860m has two chip types, GK104 and GM107. The GM107 version is *exactly the same* as 960m. Well, not 100% true, but very close. So, a 4gb 860m with GM107 will be better than the 2gb 960m.
But, there is also the GK104 version of 860m. And it gets tricky to say which is better. GM107 is a newer architecture, and will handle some things, such as tesselation immensely better. It will also age better. But GK104 is a physically stronger chip, and can simply bruteforce some problems that GM107 can't.
In the end, I recommend the GM107 (aka the 960m) since it runs much, MUCH cooler than the Kepler series, and for the laptop that is the thing that matters the most.
Fuck nvidia naming.