Typically, stools with backs are the most comfortable and if you spend a lot of time on them, they provide support on the back.
Our Eva Barstool has a back that would be much more convenient if you're socializing for an hour or more at the bar or counter. If it's just a place to sit down to have a cup of coffee, though, or just to sit for a short time, bar stools work great.

Likewise, tufted padded back stools are a nice touch, which, compared to a metal back, can be more comfortable. Some people don't like the feeling on the back of cold steel, but we noticed that it's not a concern most of the time because the stool's comfort makes up for that.