Keys and your wallet in your pocket??? A definite NO!

If you still manage to create artificial hips then a shoulder bag to redeem yourself would be a great idea, and they're very different from man/sling bags. A sling bag might be used for an outfit to look more authentic but either way, I'm not a sling bag fan but more of a shoulder bag one.

To differentiate between the two:

Sling bags are small while shoulder bags are bigger.
In a shoulder bag, you can put your work in it, keys and a wallet and even a magazine (GQ, Destiny Man or Men's Health).

Shoulder bags are smart and they make a person look more professional whereas sling bag do the complete opposite.

Shoulder bags are mostly used by upcoming young modern man who have just fallen under the working class or on their way to it.

Very often I've had to argue and teach people about the convenience of a shoulder bag. Shoulder bags can be used to put your wallet, keys, sunglasses or whatever that you may use. 

So, if you're still not convinced about the idea then leave shoulder bags to those who love them and stick to putting your keys and wallets in your pockets or continue using your sling bag (if you have one).