Slings are the bastard children of shoulder bags and backpacks. I use one for my Leica because the camera and lenses are small and light. My other cameras are full size and I can never get what I want in a sling so while it is convenient to wear and carry, that I cannot hold what I might want makes them a non-starter. Despite lovely marketing, I do not find them so convenient to get stuff in and out of quickly either.
Thus at current course and speed, I am using backpacks, and the one I use most is the Streetwalker model from Think Tank that has wheels. For me it’s the most versatile although I must be disciplined to not overload it with crap that I don’t actually need and be willing to zoom with my feet more.
Do be aware that many Euro carriers don’t allow carryons AT ALL, which sucks and blows simultaneously. In this case as I carry medical equipment (a Sleep Apnea machine), I put the body and one lens in this bag and thus far have been ok. Air travel is a consistent pain in the ass and is not getting any easier.
On a related topic, regular reader Wilf made a comment that many makers advertise their bags as not only being perfect for your camera gear, but also for your keys, wallet, passport, phone etc. I cannot more loudly caution against this. Thieves know what a camera bag looks like and even if you do as I do and put gaffer tape on the bag to cover logos and to make it look like it’s been through a war, you are a target. I have been robbed (in Barcelona where all my gear was taken in seconds) and the last thing you want is to lose your keys, your passport and your wallet. Keep those things in hidden pouches on your person NEVER in a camera bag, unless you are perfectly ok with having them vanish.
Finally, if you do travel, or even if you don’t get rid of the strap that came with your camera that advertises the make and model. Who are you trying to impress? It’s a great way to attract thieves and no one else actually gives a shit that you have the newest hottest whatever. Whether you prefer a narrow strap (I do), a hand strap on the body, or a wide strap that lights up and can be seen from outer space is up to you, but stop marking yourself as a theft target.
All the images are of bags from ThinkTankPhoto. This is the ONLY brand that I recommend. You should buy whatever you like. I don’t look at or care about fashion bags, function is all that matters to me, so distressed leathers, shiny buckles and bags that look like 1890 Africa Safari stuff are never in my personal wheelhouse.