QUANTA!! contrary to popular belief, astro is in fact harder ( the tests ussually are, perhaps not the course)
And well, Quanta is like, one of the more important physicsy type things... mabye if you like chemistry you'd like Q2Q even more.
But actaully, the choice comes down to you.. Astro can be alot more fun for most people.
Here's my summary of the two topics:
Astro: Looking at big things too far away to see.
Quanta: Looking at small things too close to see.