Most bags that are shoulder specific are designed to go over the left shoulder. I've been carrying ambidextrous bags for decades now, and I always use my right shoulder. I never thought about it, that's just how I like to do it. The only problem is now I want a shoulder specific bag, because they fit better, and are easier to adjust the strap length on the fly. the problem is that because I'm "left swing" I have fewer choices.
My advice go with what's comfortable, but if all else is equal get used to wearing it over your left shoulder, and you'll have far more options down the road.