This is a good question, and the differences are pretty small. One difference, although I'm not sure what the impact is of this difference, is that if you say

This is a good question, and the differences are pretty small. One difference, although I'm not sure what the impact is of this difference, is that if you say you've forgotten the key, that's an acknowledgement that there is data there to be discovered, just that you can't provide access to it. Does this leave a door open to other efforts to decrypt it? I don't know.