I used zip locks for our first two trips. The gallon size work great for a kid's shorts outfit. I did get a pack of the two gallon size for the grown ups stuff as well.
We've gotten a bunch of the roll up bags as "gifts" from our credit card company. I've had problems with them blowing out, but I have used them for local travel.
I liked the convenience of the bags because the kids are already matched, and no question of where's my socks, etc. Even with the ziplocks, it does make lots more space.
Good luck, and have a great trip.