The only reason to use a messenger bag is if you're a messenger or if you get into your bag multiple times while you're riding. I think that most cyclists use messenger bags as a fashion statement or it's what they prefer. There is no mistake if you're comfortable. A bigger mistake would be spending money on a bag you don't need to impress people you don't know. Stick with what you have and if you need to carry more stuff, think about gettting a rack and some panniers. Since you're riding a hybrid it's smarter to let the bike carry the weight and give your back a rest.

If for some reason you absolutely must have a messenger baq consider one of the smaller manufacturers.
I use a Bailey Bag, it's waterproof and made in New Hampshire: http://www.baileyworks.com/
As far as slipping goes I know most bags have a chest strap to prevent this, but most riders get to where they can hold the bag high on their back without it slipping. After 7 years of messengering I think my spine is permanently curved in the shape of my bag. But if you have an upright riding position it makes no sense to have all of your pack weight concentrated on one shoulder.

Again, my vote is stick with what you've got and save your money for more important stuff, like a nicer wheelset.

-Marcus