Both are valid. It depends on the footprint, type of SaaS (e.g. running complex offline-processing vs running heavily customized web-shops), your PII regulations and your ideal customer type and size.
It can be easier to have custom subdomains running separate instances, since this will limit how outages/upgrades/security affect customers. You also can intentionally keep a customer on an old version, which can be a bug or a feature.
But it can also be easier to have tenants, since you can update all users at the same time and you will pay less overall for infrastructure.
Having done both, I personally prefer tenants since it's easier to write, support and scale. But if you're B2B, you should carefully investigate the other option.
It can be easier to have custom subdomains running separate instances, since this will limit how outages/upgrades/security affect customers. You also can intentionally keep a customer on an old version, which can be a bug or a feature.
But it can also be easier to have tenants, since you can update all users at the same time and you will pay less overall for infrastructure.
Having done both, I personally prefer tenants since it's easier to write, support and scale. But if you're B2B, you should carefully investigate the other option.