The Windows 10 edition is for the Microsoft store only. It does not use Steam. Therefore, you can't use the Steam key and you must buy it directly from the MS store.
It comes with After Dark, but is very limited. No workshop, no map editor, no asset editor, files are hidden and encrypted. No easy backup and has connection issues as of last week. I got tired of losing my game, so I'm waiting for them to work out the bugs.
Also, Tantulus Entertainmemt ported it and not Colossal Order. It is the same as the Xbox One edition and not keyboard/mouse freindly. It works fine with keyboard/mouse, it just can takes extra clicks to do things.
It does seem much more optimised than the Steam version, lots less lag and things load up quickly and plays fairly smooth. Likely a lot to do with no Workshop interaction from Steam. It uses a lot less memory. Cities.exe loads up at under 2gb of ram. It will climb as you grow, but you start again, it eill be back at just under 2gb RAM use. Maybe a memory leak?
I enjoyed it very much despite the setbacks. I normally plazy vanilla anyway, so the workshop missing didn't bother me too much, although I couldn't use my info mods to peek on the balance of my city.
But if you already have the game, it will be pointless to buy it. The miggest plus to it is that it doesn't have the workshop to break games every paztch. The downside is also the workshop as you won't get access to the thousands of workshop content available. Hopefully someone can hack them in, if anyone bothers since you have the azbility in the Steam version.