I just got a KU3700 with the 5200 belt back in May. I can't say the 5200 belt makes a difference because I have no point of reference, as I never had the 3700 belt to begin with...but...I do have a G1 Late Season (with the UL stays) which sports the wrap tech suspension belt (same as the 3700). The KU is noticeably more comfortable to me compared to the LS after 50lbs or so. Now, understand I have only hauled 65lbs of rice in my 3700 thus far during my training hikes, and the LS doesn't have the precision lift plus built into it's suspension. I do believe the precision lift makes a substantial difference, probably more so then the belt! Also, the KU has yet to see any action in the woods.
I too thought about the 5200 as dotman suggested, but after talking to Aron and Sawtooth (Paul Vertrees, he helped Patrick test the KU's and he still has the prototype 3700) and doing some research, I opted for the smaller pack for several reasons...
1. The pack is way more spacious then it seems, mostly because it's a top loader with the compression lid. I was really surprised how much gear you can stuff into the bag. I can fit my 7 day hunting load out, including food, in the bag with no extra pockets.
2. It expands easily with pockets. If I need a maximum load out, I can add on 2 long pockets and a medium pod. With that pocket configuration, the pack still comes in under 4lbs.
3. I was told (and have come to find) the more stuff you fill the pack with, the better it stabilizes the load. The same held true with my LS, but getting the 3700 nice and tight requires it to be stuffed full. The LS seemed easier to tighten down when it was not full.
4. I don't want to carry more then 80lbs, but if I have to I can using the 5200 belt.
5. I went smaller because I don't live out west. If I did, I would probably have a Timberline (or 2) of some fashion. I only hunt west of the Mississippi for 7-10 days a year. This pack will be used mainly in the Midwest camping or hunting for whitetails.
6. I'm kind of a gram weenie, the smaller pack forces me to keep my gear to a minimum...
I really don't think you could go wrong with either pack, it's just a matter of preference. Lawnboi, I'm in Milwaukee if you want to check out my pack...