Some of the commercial SSL Certificates offer specific features that Let’s Encrypt does not, such as a validation of business or extended validation, while others offer WildCard SSL. The type of certificate doesn’t better or worsen SEO or SE rankings, nor are they are more or less secure.
The certificates from Let’s Encrypt are valid, and serve their purpose. Unless you need the features a commercial SSL Certificate offers, or you simply don’t want to renew every 90 days (which, from a bit of reading, may be reduced in the future, meaning you need to renew more often), there’s really not a specific reason to use them.
If you’re running a business, an extended validation or business validation SSL generally provides the “green bar” instead of just green text in the URL bar as well, though requires that you’re a legally registered company and are able to verify that through paperwork or other documentation.