Another Lowepro Flipside owner here -- the 300 and 400AW in my case. However, I never swing the backpack around while it is still attached to my waist -- I just take it off when I need to get something out of the pack. The 400AW holds my 7D2, 5D3, 100-400L II, 24-105L, 16-35 f/4 IS and 40mm pancake, plus my EOS M and two EF-M lenses, yet is still just barely small enough to fit underneath an airline seat if needed. I kept the Flipside 300 for times I wish to travel lighter and can use a smaller bag.
I tried a Lowepro Slingshot, but found that the pack really hurt my back and neck on longer hikes when it was loaded heavy. The single shoulder strap did not distribute the weight evenly enough.