I agree with everything above, but particularly that going somewhat bigger and more durable is good. Most packs have plenty of compression straps, and a bigger pack that carries well can instantly become a smaller pack that carries well. But it doesn't work the other way around, so err on the big side.
Packing everything inside an internal frame pack: I too think it's essential. Where inside and in what order you pack things affects load balance, and balance is the main comfort advantage to an internal frame. That balance will be upset by anything heavier than drying socks and a rainjacket hung off the packbag. External frame packs are fine with heavy things lashed onto the frame (not the packbag). You'll see plenty of people on the trail with heavy things like bear cans or bulky canvas sleeping bags hung outside their internal frame pack and be tempted to think it must work if they're doing it. But it doesn't mean their pack wears well that way! I think that's the main reason to err on the big side in volume.
Be wary of thinking "it's only few ounces," yes, but with one exception - the weight of the pack suspension and frame. Better suspensions and frames usually weigh more, and the extra weight there can mean a pack that carries more weight more comfortably. If the suspension is built right (and adjusted to fit you), you won't notice the added weight of the heavier pack much. But suspension and frame weight are probably the only places you can get away with thinking like that. When a suspension and frame are too heavy is a personal call, and not always so easy a call to make. So does the Circuit's extra weight benefit you?
I think you should try them both and test 'em head-to-head to find out. People are different, and a suspension that works for one person may not work as well for someone else. You'll want to get the suspensions set up to fit you properly so you don't misjudge either pack based on wrong set-up. Get some expert set-up help if you can find it. If you have to mail order, I think you should look into return policies, order both, and plan on sending one back. (It's probably more fun that way, too!)