I'm an Olympus shooter and use Lowepro bags - I have 3 sizes according to which gear I am taking for my chosen targets.
The ProTactic 450 AW II, Flipside 400 AW II, and lightweight Photo Sport BP 300 AW II. The 'AW' stands for All Weather and they truly are! The 'II' versions have relatively minor but very worthwhile detail improvements and the 'AW' have a permanently attached roll-away waterproof cover - Well tested by me even on wet sand beaches on the ground while I shoot surfers from a tripod for example. While mounted on your back no thieving hands can open the bag, so they are very secure. I have kept my 450 AW version 'I' for storing camera gear at home - It makes a perfect 'box'.
The 450 is no problem fitting in aircraft cabin overhead lockers. I am usually carrying about 10kg in it while flying to my base destination. The bags are tough and designed by a couple of American serious hikers who are also photographers and so they know what we want!
I only use my Flipside 400 when going out with just one M1X body and Olympus big white supertelephoto. I never use the flipside feature as I prefer to always have a camera at the ready on a Peak cross chest strap - I find all flipsides cumbersome and noisy when shooting wildlife.
The lightweight Photo Sport is superb for carrying clothing, food, torch, etc and even a small Jobu gimbal plus Skimmer ground pod. It also has a sewn-in stiff padded side access compartment for a camera and/or lenses such as a macro or wide-angle < Watch the extremely thorough video review posted below.
I often carry two M1X bodies, each with a lens mounted on Peak chest straps and my blue (available in black) 300 on my back with an extra lens and all the clothing and food I need for the long day.
View: https://www.youtube.com/watch?v=fx_Zr3Wvv1g
As with all gear, it depends what you photograph and where and when and each of us are different!
WEX are able to offer Lowepro bags at very competitive prices because they have a special relationship with Lowepro-Gitzo-Manfrotto who are one company. Also, if you buy a bag and it's not right you can return it - I did that with a 600 AW thinking it would be good for my Canon 500mm when I had Canon gear.
HTH