Barrels are much more waterproof and reduce food smells. Although barrel harnesses have come a long way, they will never be as comfortable as a canoe pack or backpack. So, here is my pick:
white water trip:
Barrels only. They may have to take a swim down a set of rapids.
Flat water trip with little portaging:
canoe pack for gear
barrel for food, which I probably wont hang from a tree at night...
flat water trip with lots of portaging:
canoe packs
Food is hung from a tree at night
As far as which pack to get, if you plan to do any portaging make sure you get one with a good harness with hip belt. My buddy bought a Eureka Storm Shield 115 liter pack this year. The first trip we were in love with it. Waterproof, great harness, lots of grab handles, etc. The second trip several holes were worn into it. This pack was not abused. I am very disappointed in the durability.
I think my next purchase will probably be a Seal line pro pack (115 liter). They are super durable and seem to have an okay harness. A little more pricey at $165 though. I have used the Sealine boundary packs and find them annoying with no hip belt. They are cheap and durable, but a pain on long portages.
If you get a pack that isn't waterproof, do not forget to add the cost of waterproof pack liners. It may be worth spending a little more money for a fully waterproof pack.
As far as barrel harnesses go, I think the best one I have used is the Trailhead one. Which I noticed is on sale:
http://www.trailhead.ca/node/800
I have a little pet peeve about barrels. They shouldn't cost nearly as much as they do. They are all recycled containers, and even marketed as such (eco-friendly recycled container!). My buddy's dad bought about 8 of them for $5 each a few years back from a recycler. Someone is making a lot of money when they charge us $70 for a recycled piece of plastic.
Hope this helps.
Rob