Quite a few apps work friendly with multiple OS's these days. For example, Microsoft's OneDrive works on Android, iOS, and Macs in addition to Windows and Window phones. It also can be set to automatically upload anything, files, pictures, videos, etc. That's a plus over iCloud which doesn't do videos and a plus over Google+, which has a stupid requirement that if you delete a video or photo off your phone it also deletes it off Google+. Google is supposed to fix that in an upcoming version of Android and move the auto backup feature to Google Drive, but until they do, it's a negative. Plus, not every phone or tablet you can buy right now will be upgraded to whatever that future version of Android ends up being.
I hated Microsoft for 20+ years but OneDrive is very refreshing. OneNote looks interesting too but I haven't used it.
Chrome, the browser, also has quite a few apps that can work with it, regardless of which OS it's on. For example, Motorola makes a plugin that allows texting from Moto phones right in Chrome, just like OS X does with iPhones in Messenger starting with Mavericks.
I haven't looked at Windows 10 at all. So many years of horrible Windows experiences, I'm not sure if I could use Windows as a daily driver.
I have Ubuntu on several media PC's, it's become a very nice basic OS but won't sync everything and few, if any, apps are available for it that work with iOS/Android/Windows/Mac.
I had a Chromebook for a while. Those are good options only for casual users, which in reality is probably the majority of people. But for anyone who needs to do anything outside of a browser or Google compatible apps, it's not the right fit.
So you're really stuck with Mac vs. Windows. If you use Chrome as your browser you get some flexibility regardless of which OS your phone is.