There's some important things to consider when buying an outdoor bar stool. For example, there's a good chance that it'll get left out in the rain - or, that someone will spill beer on it. But either way, your new bar stool is going to get wet, so you'll want to buy one made from water-resistant materials. So besides picking out a bar stool which looks nice on your patio, you'll also have to decide whether you want to get a wooden bar stool or one that's made out of wire.
It seems like they'll both be uncomfortable. (For example, I saw one outdoor bar stool where the seat resembled a slice from a tree stump, which had a nice "outdoorsy" appearance but looked like it'd be painful to sit on.) Fortunately, many stools come with a separate cushion made from cloth. (Just remember to bring in the cloth before it rains, or the cushions will end up getting musty!
I saw one patio bar stool with a chair which could swivel in a circle. This makes it easier to talk to the other guests without having to move on the stool - but of course, at nearly $600 this stool was much more expensive than the simple one-piece units. Other "barstool chairs" offer more chair-like comforts, and there's even a few where the chair is made out of soft wicker. My favorite was called the "high profile director's chair," which was high enough to be a bar stool - but it was made out of bamboo!
But remember who's going to be using your outdoor bar stools: a bunch of your friends who are enjoying a barbecue. They'll be focused on the meats you're cooking, possibly drinking, and definitely having a good time. So they probably won't pay as much attention to the chairs as you think. It's probably more important just to make sure that you have enough chairs!
It's sometimes nice to get a matching set of stools, so think about how many guests you're expecting to have, and then price the chairs accordingly. There's a couple of stools which will fold up for easy storage, and that's definitely something to consider if you're going to be buying a lot of them. Of course, if a guest finds one of the stools uncomfortable, then they'll obviously find all of them uncomfortable. So sometimes it's nice to have one or two "extra comfy" chairs around - just in case your guests don't like your patio bar stools after all!