I don't really like Green Day's new music at all, but I've heard their live shows are great.
Yellowcard, on the other hand, is
awesome. I've never seen them live (my parents are deadset against concerts *cry*) but I have all their albums and I absolutely love them. The two other big YC fans I know both say they're great live, and from watching the DVD it certainly looks it. God, what I wouldn't give to see Sean do one of his backflips...
*ahem* If you want to buy Yellowcard music, I suggest getting at least the Underdog EP as well as Ocean Avenue. Ocean Avenue is good, but it's sort of... I dunno, tame? A couple of my favourite songs (Avondale and Rocket) are on Underdog, and since it's an EP it wouldn't be too expensive. If you have extra money lying around, you should also buy One for the Kids, which is fabulous, but older. Don't bother with Where We Stand. They won't play anything from it, and it has very little connection with the way they are today.
Yellowcard's new album will be released in October! ^^ I'm so excited that I'm going to hyperventilate just thinking about it.
Anyhow, if you end up going to either event, I'm sure you'll have an awesome time. I'm jealous!