differences - Would vs Will in future events

What is the difference between the following:

1. Sorry, mate. I wouldn't be able to come for dinner.


2. Sorry, mate. I won't be able to come for dinner.

Answer

In most cases, (2) would be used to say that the speaker was unable to accept the dinner invitation. (1) is unlikely to be found on its own. It needs some kind of explanatory setting, such as ‘Yeh, it would be great if we could get together some time. I wouldn't be able to come for dinner, though, because we have a problem getting a baby-sitter.'