I've just gone through the trials of fitting a pack to my somewhat rotund body, My issue also being a slipping belt. See:
http://bushwalk.com/forum/viewtopic.php?f=15&t=10306. Extreeme retching gives you functionality, but is uncomfortable and eventually painful. We have different issues but I'm guessing similar experiences. So... I might be able to provide some insights on the aarn pack, which is what I ended up getting. I will caution I've only started testing this pack, but so far am very, very happy with it. A slipping pack was the bane of my existence.
It's also probably worth you mentioning what you intend to use the pack for (Ie. Day walks, overnighters, week long expeditions, world domination etc...) and any deign particulars that are a must for you. This will also help narrow the field and give users here some parameters for recommending gear. For me, I placed fit above all else.
So some observations of the aarn that may help...
- When trying it on, don't be afraid to get hands on with an aarn, you can't break it, and it's better you naff the fit fiddling in store where someone can help you than at home afterwards. Remove the hip belt, take out the metal back supports, play with the elastics on the hip belt, remove the hood etc etc...
- The belt I find tends to sit a little high in comparison to other packs, this works a treat for me but may be a pain for others. That said the belt is very adjustable, so you can bring it in/out and raise and lower it somewhat. When trying it one, adjusting the belt will be a must for you. It is also very hugging, it molds to you unlike any of the other packs I've tried. Even if it's not a pack for you, I would be very surprised if you didn't feel 'snug' in one.
- The compression straps on the belt are dual straps enabling you to tighten the top and bottom of the belt on both sides. Although they're meant to be done evenly, I find this feature helps compensate for an odd body shape... perhaps something that benefited me more than it would you.
- I found that with a conventional pack, having all the weight pitched off my back, there was too much pressure pushing loading onto the rear of the belt which resulted the pack sliding down as I would cover ground. The aarn at worst balances these forces out. So much so that I notice the imbalance when I'm carrying more weight in the front than the back, or vice-versa. I have no slipping issues now.
Byrnesy is right, getting a good fit is crucial and there is a lot to this pack, so you will need to rely on the sales staff to help you through it, allow lots of time. I'd recommend backpacking light for this, although I'm not fond of the service I have received there, their knowledge of these packs is well reputed on this site. I sourced my pack from outsports and found their understanding of the aarn somewhat limited. It left me having to do a lot of fiddling with the pack in the weeks following purchase.
As for the suggestion to go lightweight, well I guess that depends on what you're carrying/planning on doing and is outside my experience, so I can't offer much help there.
I'd suggest trying an aarn, at worst you'll get an idea of what you don't want, and at best you'll be a happy camper like myself.