What is the difference between a Free SSL Certificate rather than a paid one?

back in the day you had advantages like the "big green bar of trust" with the company info etc. but that's now long gone.

the biggest advantage in a paid ssl cert is that you can buy it for "x" amount of time.
let's encrypt HAS to be revalidated every 3 months. that can make some difficulties in setups with reverse proxys or for example cloudflare if you want to use the "strict" encryption.

I personally just buy paid ones when I don't want to fuck around with the 3 month revalidation peroid.