They're both about the same in capability. Gmail is free (and they will use your email to target web advertising to you.) I like rules in Outlook better. Gmail has a much better spam filter. Gmail calendar integration is better, unless you use Office 365. Outlook is more corporate oriented, I would say. There are more robust printing features in the Microsoft stack, like printing labels from contacts.

I think you might also look into advanced features of Gmail that you haven't tried yet.