You really have to decide for yourself. It's a bit a matter of taste and a bit a matter of your translation workflow.
Real messages are good when you don't want the overhead of maintaining an additional translation file (for the origin language). Furthermore, if you forget to translate some of the messages, you'd still see a valid message in the origin language. It's also somewhat easier to translate from an original message rather than a keyword.
Keywords are better when messages are changing often, especially with long texts. You abstract away the purpose of a message from the actual text.
EDIT: there's one more scenario when you could argue that real messages are better than keys - when your website only supports one language but with multiple variations - like en_GB, en_US. Most of the messages will be the same, only few will vary. So most of the messages could be left as they are, and only the ones which are actually different between GB and US put into a translation files. It would require much less work compared to an approach with using keys (of course, assuming your messages don't change very often).