Hi Lin, both packs you’re deciding between are very solid choices. In fact, they both made my favorite backpacks list, so you're definitely on the right track. I think your decision should ultimately come down to your backpacking style (or the style you’re striving for) more than anything else.
The Arc Haul is a fantastic pack for ultralight hikers, but if you try to carry too much gear it won't work as well, in my experience. They say it can carry up to 40 lb, but I’d view that as more of a max weight limit. For optimal comfort you’ll probably want to be around 30 lb total or less. A base weight under 15 lb would be prefered and around 10 lb would be ideal. So, if you're a thru-hiker looking to minimize weight and bulk on the trail, the Arc Haul would be a great choice. Yes it does cost more and you’ll certainly want to add belt pouches at very least, and maybe a shoulder pouch and ice axe loops too (I recommend all three), which will increase the cost even more, so that’s definitely something to think about. You’ll also note that it carries significantly less volume than the Circuit, and that’s by design. True minimalists don’t need that extra space, so if you think that’s you or what you’re shooting for, the Arc Haul is a great choice.
If you're transitioning to lightweight backpacking or you just want more room for luxury items, I'd go with the Circuit. It does weigh about a pound more, which is a lot for a thru-hiker, but it also has a lot more room for gear and will carry heavier loads more comfortably. The Circuit's frame is sturdier, the pack costs less, and you won’t have to order any extras like hip pockets or ice axe loops. It’s still light enough for a thru-hike if you keep your other gear light, but serious minimalists would probably go with a lighter option. Also, if you do choose this pack and you’re on the fence about torso sizes, I’d go with the longer size because I think they run short, but that’s just my opinion.
As far as the back panels go, in my experience the ZPacks back panel design only works well if you’re carrying smaller loads (think 10 lb base weight max). If you jam the pack full of gear it tends to bulge out and press against your back, which will minimize any airflow effect. That said, if you keep your weight minimal, the back panel on the Arc Haul is a nice benefit. The Circuit has a very comfortable back panel, but you won’t get any airflow. Honestly, my back is usually pretty sweaty when I hike one way or another, so I don’t find this to be a dealbreaker. I think comfort is a more important factor, and the Circuit has that going for it.
Also, you may have already seen it, but here’s a running list of my favorite backpacks and critical pack considerations, which might help with more details.
Have a good one and happy hiking!