I've got a large Arkel handlebar bag. It is a bit on the heavy side, but a great performer. The zip-out liner is handy when stuff gets spilled (or you forget that you had a banana in there and it gets destroyed). The zipper has worked without any problems for a couple of seasons now. I had a cheap Cannondale bar bag years ago and the zipper failed within a few months. The map holder is useful, but not actually something that you can use while riding. You'll still need to stop and step forward to lean over the bag, and sometimes to just remove the map from the top slot entirely.

One characteristic that was important to me was to be able to open the main compartment while riding, which means that I need a bag with a lid that opens away from me. Most bar bags open towards you. That makes accessing the bag while pedaling very difficult, and sometimes just impossible.

"Waterproof" bags are overrated. Water will seep in there anyway. And the rubberized bags just hold moisture in, never letting it evaporate. My advice is to ignore claims of water resistance. Just get a normal bag and be sure to wrap electronics very securely and keep clothing and such in plastic bags (trash compactor bags work especially well).

The lower-mounted bags (sometimes referred to as "porter" bags) look like they would offer greater stability, but in practice it seems to have no effect. Setting the bag lower does allow you to have a larger bag that doesn't extend too high. A bag that rests just a bit above the handlebar will make accessing the contents while riding much easier. If you don't intend to access the bag while pedaling, though, that may be of no value to you.

Another thing to consider is a small bag that mounts to your top tube, close to the stem. That is a good place to store snacks, sunglasses, a small camera, or other very small items that you would like to be immediately accessible.