Never tried the peak design, I have a number of the Think Tanks Retrospective canvas bags that I really like, it's my around the city camera bag. I agree with you on the Peak design clip system, I picked up their Clutch handstrap to tes with my Sony body and it's really well thought out.
I've got the retro 7, fairly small, but massive compared to peak sling 5L. Retro bags are also too heavy, I feel it after a day of use. I also have smaller bags that can fit a similar amount to the peak, but the design and fit are better with the peak.
One of my favorite shoulder bags is the lowepro passport sling. I really grew into the bag despite feeling kind of cheap initially. Similar size to the Retro , but feather light.
If I overpack that little bag, then I do belt or backpack. I won't tolerate discomfort.