I'd tend to agree with PC2K, the pack (with it's packstraps, grab handle, gear loops, etc) would be easier to hold on to, and there's something to be said for the ability to wear it on your back once you're out of the plane, leaving your hands empty and free while you're moving. Waterproof bags inside a pack also give you a surprising amount of bouyancy if you end up "in the drink" (see any field manual for ALICE gear ruck and liners or try it yourself). Regardless of how tight you pack the waterproof bags/liners, unless you vacuum pack them, there's still a lot of air space inside of them, a 60 Lb ruck will float itself and have plenty of flotation left to share with you.
Just my two cents.

Troy