If you are completely new to the cloud, both of these are good first certifications. They cover similar ground — core concepts, key services, pricing and security basics — at a similar price and difficulty. The decision is almost entirely about platform, not the exam.
Pick by platform, not by exam
The exams are close enough that the tie-breaker is which cloud your target employers run. AWS has the largest overall market share and the widest range of jobs, so it is a safe default if you have no preference. Azure is the standard in many Microsoft-centric enterprises and across much of the European and DACH market, so AZ-900 is the better fit there. If you already work somewhere, the cloud your employer uses is the obvious choice.
Cost, time and validity
Both cost around $100 and take one to four weeks of part-time, non-technical study. The one structural difference: the AWS Cloud Practitioner is valid three years, while AZ-900 does not expire — a minor advantage for Azure, though most people move on to role-based exams regardless. Neither should be chosen on validity alone.
Who these are really for
These are foundational, non-technical certifications. They suit beginners, students, and people in sales, project, recruitment or management roles who work alongside cloud teams and want credible cloud literacy. For an engineer aiming at a cloud or DevOps job, many skip the fundamentals entirely and go straight to the associate level (AWS Solutions Architect Associate or Azure AZ-104).
Think one step ahead
Whichever you choose, the foundational exam is a stepping stone. On AWS the natural next step is the Solutions Architect Associate; on Azure it is the Administrator (AZ-104). Because the concepts transfer between platforms, the second cloud is much faster to pick up later if you ever need it — so there is little value in taking both fundamentals exams.
The honest answer
Pick the platform your employers (or target employers) use, do that fundamentals exam if you are new or non-technical, and then progress to the associate level on the same platform. If you have no preference at all, AWS is the safer default on market size; AZ-900 is the better fit in a Microsoft-heavy or European context.