I generally discourage off body carry. It has to be done correctly to be able to deploy the weapon and to minimize the risk of losing the weapon.
On the occasions that I do, I have a Maxpedition Jumbo LEO Versipack. I have their holster, which I adjusted for my Glock and has held every gun I've put in it, without needing adjustment. It's probably one of the better ways to carry off body, because it puts the gun right where it needs to be and even if you're looking at it, you wouldn't know it's there. There's more than enough room to carry whatever you need; I've even stuffed gym shorts & a t-shirt in it. Of course, I couldn't put my work clothes in it after I changed...
Whatever you use has to be securely on the body. That's one of the reasons I like the Maxpedition; the strap is designed to go across the body and it has a "belt" strap that you can use to secure it to your waist. I've never felt a need to use it, so it's removed and folded away.
Ultimately, it depends on when/how you plan to use it. That's what will determine what will work best for you. The ARES looks cool, but I'm sure it's $$ and I don't carry a backpack enough to use it. I'd have to see it in person and try it out to be comfortable that there was nothing that could go wrong during deployment. One thing I don't like is the deployment; there's no way to be discreet about it, should the situation call for it.
