I'd go with windows 7. However, being smart, I'd also go with a 2nd partition you do nothing with for windows 10. just make sure you set updates in 7 to manually select or you might wind up with a "upgrade" to 10. The reason I say it that way is that the window for getting win 7 updates may end sooner than we think by MS "forcing" people to upgrade to keep security updates coming AND them moving win 10 to a recommended update. If you do decide 10, get something that brings back the start menu from XP-even if it costs money, it's worth it (if you go with 8.1, GET SOMETHING THAT BRINGS BACK THE START MENU! the UI is for a touchscreen!)

Makes me wonder if driver differences between USB 2 and USB 3 are even different at all, since at the heart of it it's really how fast the hardware works...

I'm still not convinced with DX12, namely you have to have DX12 games for it to be relevant, and most games are going to be DX9 or DX11 (as DX10 flopped with Vista never taking off :P and to think they pushed Halo 2 on it too...); With a number of games, emulators, indie games and the like I see the same patterns of hardware that's supported, namely DX9 (compatibility), DX11, and OpenGL (for linux port?). So unless there's really a couple must-have games that only run on DX12 I'm not sure if you're getting any advantage at all.

Win 10 can be a bit problematic depending on your setup at the moment. The pro version allows you to turn off some stuff that the home version does not allow. So if you tolerant of driver issues, have the hardware for dx12, and can get the pro version of windows 10, then go with win 10 64 bit. In all other cases, go with Win 7.

On a base level it's hard to directly perceive the difference (especially when only the mobo/system support it but the pendrive or plugged hardware don't), but when you need to move/copy/backup huge chunks of data then you can see the advances.

Agreed for the most part, but just because DX12 isn't fully out yet, and DX9/10 are a thing of the past. Another thing, the directx features from now on comes mostly "under the hood" than anything (better use of GPU/CPU resources and that sort of thing) and you mention emulators, I like to follow some news about some of them and since the introduction of DX12 you can see a lot of improvements regarding optimization and such.

Except from what i understand the USB was never originally intended for storage or data transfer, it was more a replacement for the printer plug from that huge bulky thing to something considerably more slim. However once the market started putting out thumbdrives (starting at about 32MB) speed started to become more of a priority. That and certain devices like webcams which have a higher bandwidth.

I don't keep that close attention to it, however i can't argue with that. Recently been reading an interesting tutorial on using multiplication to do division, and using shift & add to do multiplication. Quite interesting stuff... Actually i'm suddenly reminded of MMX extensions, and how using interesting programming you could effectively do something like 64 addition operations in a single instruction. (Mind you, you have to fill all the MMX instructions, and they all have to be 8bit variables packed, and the adding wouldn't overflow affecting the next variable, etc).

I think there's more to it than just 'under the hood', but i don't know/have those details...

Just like any other OS.

I'd say that it somewhat depends on your hardware, but in general I'd say: try 10, and if after a few weeks you don't like it, go for 7. I simply think it's better to use the latest OS if you can stomach it, and that 10 and 7 are both far enough from XP that moving to 10 won't be that much harder than moving to 7.