It depends what kind of site you run
Only one of the two standards applies to everybody. UCP describes a business selling goods, so it has little relevance to a publisher and none to most SaaS products. WebMCP applies almost everywhere. Each sector below sets out which matters and why.
Ecommerce
Agents are already buying. The question is whether they can buy from you.
SaaS platforms
Your users will start sending agents into your product
Travel and hospitality
Booking is the hardest thing to hand to an agent
Publishers and media
For you this is about access, not commerce
Marketplaces
Search is your product, and it is what agents handle worst
Not listed, or somewhere between two of these? The free check works the same either way, and tells you what is actually on your site rather than what ought to be.