I believe both are perfectly valid, so it should come down to the difference between entities and Unicode characters.
HTML Entities are the safe way to go. They will show up on all modern browsers, but are a bit of a pain in source code. Plus, it's several characters to represent a single character, which isn't as efficient as it could be. If you were going to go with the entity, I would recommend using
© instead, for readability.
Embedding the copyright symbol should work just fine, as long as you have your encoding set correctly. However, if you run into encoding issues, you can get all sorts of display issues that are unsightly and hard to deal with. If it's in your meta tag, then you shouldn't have display issues, but it's something to take into consideration.
I usually use the single character. That's what it's there for, and it's a couple fewer bytes off of each page load.