AWS is the market leader whereas Azure is quickly increasing its revenue and user base.
It is a matter of preference in terms of the piece of work that you want cloud computing to address.
AWS leads by a great margin in terms of IaaS(Infrastructure as a Service) while Azure leads the pack in terms of PaaS(Platform as a Service) and SaaS(Software as a Service) offerings.Microsoft has a host of its offerings on Cloud through PaaS and Saas be it the SQL Server, Office 365 etc and is increasing its IaaS business.
I’d suggest that you make a choice in terms of choosing them based on your preferences, however, my humble advice would be to learn one of them in good depth and then start working on the other.
It’d make you a cloud developer instead of just Azure Developer/AWS developer.