IMO, they way gettext handles translations is way superior than the key->value model for many reasons:

1. You never will run out of keys.

2. Makes code much easier to read with a real language on messages instead of keys that can't possibly describe the whole message.

3. While writing code, you don't need to stop to think in a new key to add your message, go to the key->value list file and add it there. You just wrap you message with _('') and it's done.

4. Easier to write messages that have variables inside using sprintf or similar.