I forward my incoming gmail to Outlook.com (Outlook on the Web), and I configured Outlook.com to send mail through gmail. So I use both simultaneously.
Outlook.com has much better support for displaying attachments of MS Office documents. There are no formatting mess-ups. This way, I don't have to download the attachments, and open them in MS Office. I just view them and/or print them straight from the web interface.
I also gain a backup of all my mail, including sent mail, because everything is on both Gmail and Outlook.
Outlook.com has a cool dark theme. Check it out.
EDIT: If you use an adblocker, you will get a nag, but you can remove it with a browser extension such as Webmail Ad Blocker.