I know a couple of people are suggesting the roll up retractable ones... As a nanny I've worked with these in several different homes with different ages of children... Now as a mother I have to say I would NEVER trust one with my children!
There's a little button that you have to push just right to get it to expand... 2 different 18mo figured out how to do this and push on it so it was loose enough they could crawl under it!! The one 2y/o figured out that he just had to push on it hard enough and the thing would click and he could climb under it!! I don't want to seem like the voice of doom over here, especially for a product that a couple people swear by!! I just want to let you know about my experience so you can make an informed decision!!
We have the Regalo Easy Step, but like someone siad it does have a part that goes across the floor and it probably wouldn't be good at the top of the stairs! We use ours at the bottom.... I would suggest something that screws into the wall!! Other then that I think it's all a matter of preference. Since good gates are expensive remember to think about the long term usefulness of the gate!!