I agree with aharrold45. I've just bought a 55litle backpack, its a special one for girls or smaller people generally. I went with the view that the bigger the pack I got, the more stuff I would take, and I'm sure I would regret taking half of it!
I never really considered a suitcase, I'd be a bit embarassed to be honest! If you can't carry your stuff on your back then you've taken too much.