MicrosoftIt depends on why you want to get the certification.
MB-210 Pros:
MB-210 is just Dynamics 365 Sales and includes Lead and Opportunity Management
There is a Practice Assessment
MB-210 Cons:
The certification will only be valid for a year
MB-280 Pros:
The certification will be able to be renewed in a year
Covers more than Sales including Customer Insights - Journeys and Customer Insights - Data
MB-280 Cons:
Covers more than Sales including Customer Insights - Journeys and Customer Insights - Data
The scope of Sales is deeper with less basic Leads, Opportunities, Quotes, Invoices
There is a lot more to study Study guide for Exam MB-280: Microsoft Dynamics 365 Customer Experience Analyst | Microsoft Learn and you will need deeper understanding of Sales, the MB-280 exam is more challenging than MB-210.
No Practice Assessment (yet)
Read these 2 posts to help you decide
MB-280: Microsoft Dynamics 365 Customer Experience Analyst » The CRM Ninja
If you don't need the MB-280 soon, I would be tempted to take the MB-210 now and wait 6-12 months and sit the MB-280 once it has time to settle down as an exam
