I would get the small in black since it would match with more outfit. I would get the mini in a bright fun color to change it up. You may want to try the mini in person first since the shoulder strap is not adjustable.
I think it depends on what you like to carry. I have both bags and a Disco. The small is a bit longer than the disco and therefore fit longer wallets. The mini is actually a little smaller than the Disco so you won't be able to fit as much.
I usually always vote for the bigger bag because you can use it more for daytime & nighttime but I tried on both bags just this weekend and I must agree that I think the Mini looks much better on the body and the Small somehow "sticks out" too much of the body because its rectangular shape.