Ask HN: If code is dual licensed (GPL & MIT) which applies when?

Actually, that's kind of a dumb question, because obviously both apply... I guess.

The code in question is a JQuery plugin which my startup wants to use for its commercial product. I'm a little confused as to which license applies, given the fact that the code's owner does not specify terms and conditions in his/her README file in the Github repo, or anywhere else for that matter.

I'm something of a noob w/r/t using open-source code, so any help would be appreciated.