It's pretty obvious which one is going to be easier given triple gives you 3 GCSEs and double only gives you 2.
Triple is naturally going to look better on an application (depending on what you are applying to), and does include 9 exams (3 each for physics, chemistry and biology) versus I believe 6 for double, so not a huge difference. They aren't too long either. (EDIT: apparently it varies per exam board now)
I'd try stick it out if you can but if not it's not the end of the world! Consider talking to a member of staff to help you decide as they'll have some good advice.