I prefer a backpack usually unless I just have a few things. For the most part I only do hiking that lasts a few hours or so so it's easy to fit everything into a small backpack or small bag.
Every once in a while I'll leave the water in the car and go solo. I hate carrying things sometimes. Plus that way I get a nice drink when I am done. I actually don't think I'll be doing that for a while though. It works in the spring but it's too hot during the times that I go out now. It would be really hard and potentially dangerous to go without water in the heat.
I usually prefer a good backpack that has many compartments, and ideally, a dry bag. And, like Andrew says, sometimes I just want to go with minimal baggage, and only put some water in my bottle holder.
I would agree that it just depends on what I am doing and how long I am going to be out doing it. I know when I go hiking I like to take a backpack but if I am not going to be gone long I will stick with the handy old fanny pack.