I'd do mechanics mostly because it's fascinating. It will add to your physics course, it goes over stuff physics doesn't and goes over things in greater depth. Pure 2 is cool, I'm self studying it and Pure 1 this year (my school didn't let us chose, we did decision 1 and mechanics 1), but it really isn't necessary for the courses you want to do, and won't add anything important whereas mechanics may very well add important stuff.