There are a lot of blind people on here, who use screen reader software to tell them what is on the screen. To help screen reader users, it's a good idea to use emoji rather than old-style smileys.

For example 😄 will be read out loud as "smile" because that's its alt text. However, :D will be read out loud as "colon D".

(Some old-style smileys do work with screen readers, but most don't. Emoji are safer.)