The pro will get you negligible performance differences, but a superior longevity and lifespan. Honestly though, unless you're constantly reading from and writing to the SSD(content creation, etc), just save your money and get the evo.
It appears that cost is no issue for you haha. but a 500gb will be fine for windows, plus a few large games. I've heard you dont want to get SSD's too full as it may degrade performance. I'd probably get a 1tb if I were you.
OR, you could just get a 250gb or 500gb as well as a standard sata SSD, which is anywhere from 1/3 to 1/2 the price for the same capacity. But its up to you!
When you say run windows 10 from a usb, you mean installing it from a USB right?
Then just grab a 500gb 970 evo, and a 1tb 860 evo. Install windows and some priority games onto the 970 evo, and keep the 860 evo as a data drive. Or you could even just buy a western digital blue standard hdd, but with your budget, I'd just go all SSD.
For data storage, I actually run a NAS (Unraid server with 9TB storage).
It's ok, a little slow especially as drives spin up on first use, and probably not for installing apps on to run on this PC, so do you think I could just get the 500GB 970 evo if I don't get the 1tb 860 evo or maybe just go for the 1TB 970 evo?