I own the Windows/Xbox and Steam version and I definitely recommend buying the Windows 10 version over the Steam version for multiple reasons (I can more into detail if you want).
I bought KI shortly after Season 3 came out and haven’t bothered to look when the game goes on sale because of that, so I can’t help you there.
As to the different Definitive Edition, I noticed that the more expensive one includes the Killer Instinct: Definitive Edition Hub. But I don’t know if it works on PC.
You see, the way I bought KI is very complicated.
First I bought Shadow Jago when he was still exclusive before I could even play the game, then season 3 dropped with the Supreme Edition.
The thing is, the Supreme Edition wasn’t available in Germany because of the 2 classic games (which I couldn’t even play because I didn’t have an Xbox One at that time.
So I bought the Ultra Edition of Season 3 + the Season 1 and 2 bundle.
Then after a while, the Definitive Edition dropped and at that time I also had an Xbox One, so I bought the Definitive Edition as a disc.
That means I can play KI (the reboot) on both Windows 10 and Xbox One and have all the content, but when I want to play KI 1 or 2 Classic or the Killer Instinct: Definitive Edition Hub I need the disc.
The Xbox One disc obviously doesn’t work on PC. So I don’t know how it works on PC.
Since you get KI 1 and 2 Classic when you buy it on PC even though you can only play them on Xbox it could be possible that the Hub is also only available on Xbox One.
In short: It’s probably not worth the higher price.