511 makes top quality gear, in my opinion every bit as good and in some ways better than Maxpedition, I had one of their 511 Rush MOAB 10 sling bags, and had the same problem with the strap riding high and chaffing my neck, the heavier the load out the worse the chaffing got.
I've since sold it and I'm in the process of trying to find a bag to replace it, in the meantime I'm using my small tactical Falcon 2 assault pack for a day bag, it's a great backpack for a day bag and can be stretched for an overnight pack, and it's suspension system is quite comfortable.
That said, it's still a backpack, my problem is that because of a slowly escalating problem with arthritis in my shoulders I'm finding it harder and more painful to get a backpack on and off because of the dual straps, a sling bag is much easier and less painful to get on and off.
The problem with sling bags is they all have a problem with strap creep and neck bite, it's inherent in the one strap design, I've tried several different types and brands, they all have the same problem to different degrees, the only one that didn't and the one that I've settled on is the Maxpedition Jumbo Versipack, the bag is the right size, has all the organizational requirements I like, fits and carries well and is firearm ready, I've had my Scandiam frame 1911 commander in it and it carries well, the one difference between it and the other bags I tested is that it comes with an additional strap that comes across the body back to front to keep the carrying strap from creeping up on your shoulder and digging in to your neck, the "keeper" strap and the main carry strap both have quick release snap buckles for easy on and off, another feature I like is that the bag can be worn on the hip or at the small of the back depending on your prefered style of carry.
About the only drawback to the bag is they aren't cheap, MSRP is about $100.00, Amazon has them for about $86.00 with free shipping, but the quality is there and the buy once/cry once rule applies.