Alright here's my take on this. I'm an ex Windows Phone user and have converted to Android 3 years ago. I've tried to get back to the Microsoft ecosystem on several occasions but I just can't because it no longer fits my needs.
So, Outlook for Android is an excellent app. It really is. But only if you use your main email address. Otherwise it's a bit more work.
Personally I have four email addresses:
@gmail.com,
@outlook.com and two custom domains. On Gmail, I can add all four of them on gmail.com and boom it works everywhere, PC, phone, etc. I have one place for all my emails, all my contacts and so on. I love that. I can write and reply from all four emails with nothing particular to do. It's perfect for me.
Now outlook.com allows you to add your
@gmail.com account or custom domain email accounts. You will be able to use them on Windows 10 but not Android. On Android, you need to have all your email accounts separated. You can stil keep all your contacts on outlook.com but your email will be split.
Outlook has a cool feature: aliases. You can have, for example,
myname@outlook.com as your main email address and, if you want to keep that private, create
thisismyname@outlook.com for example. BUT if you get an email sent to
thisismyname@outlook.com and you want to reply to it, you have to manually select the alias in the "From" field everytime. It doesn't select it automatially and there is no option for that.
So it's a question of how you use your emails and also what ecosystem you use. If you like and use Google apps, Google Drive and so on, then a Gmail account fits perfectly in that. And you can still use your Outlook email address in Gmail and use it seamlessly. But it gets trickier if you want to use your Gmail account in Outlook (unless you separate the two).
If you're mainly in the Microsoft ecosystem (OneDrive, Office and so on) then you'll have a better experience with Outlook. But again it depends on how you use your emails.
I hope this helps!