I recently wanted a new smallish day pack with no load hauling requirements and these are the two I considered. I ultimately went with the Sitka and am happy with my choice. To me, the biggest differences are the air mesh back on the Sitka and the water bottle pockets on the kuiu. I really like the gap between the bag and your back on the Sitka, it vents really well and is comfortable.
Plus I personally like the subalpine pattern better.
One thing a lot of people aren’t aware of because they don’t really advertise it, is that the Icon Pro 1850 can be used as a day pack with either suspension system and without the frame. It comes with a pair of aluminum stays that can be added inside the bag.
I used mine with the frame and a dry bag in the meat shelf to carry food and camp for a 7 day elk hunt. Drop the camp bag and you’ve got the perfect day pack that can still haul meat.
Ditching the frame would be nice for scouting trips and general day hikes, or hunting from a tree stand.
My take on this is:
When i'm out hunting I'm intending on harvesting something. So...I want to carry a pack so that i'm prepared for the first trip back to the truck (with meat/antlers etc). The last thing I want to do is to make a trip to the truck just to get a pack for the packout. No reason for extra miles on the tired feet.
I have used the KUIU Ultra 1800 for the last 3 seasons and love the pack. It's very light and worked well hauling elk quarters, whole deer in one trip etc. It's comfortable for a day pack...the only time it's "ackward" is crawling with it on during a stalk. I usually drop the pack at that point.
One BIG consideration in my mind is how much more comfortable it is to attach bow or rifle to the pack for carrying it. There are good options out there for quick release (I use the stone glacier rifle sling, modified...and the kuiu bow holder). Without a frame pack carrying a gun is uncomfortable.
For scouting, a frameless pack is a good option IMO.
I run a Kuiu Ultra 2200 on my Icon Pro frame/suspension when doing day hunts. Nice being able to pull the 5200 off and throw the day pack on but still have load haul capability.