DD will be crawling soon and we need to think about a stairgate. Our upstairs is small: you get to the top of the stairs, there's a tiny landing about two feet square and bedrooms (ours and hers) off to the left and right (wall straight ahead). We're in a quandry about where to put the stairgate: across her doorway (seems mean to sort of imprison her in her room and will make closing her door fully impossible) OR across the top of the stairs (paranoid fear of her or one of us leaning on it if it's not closed properly and tumbling down the stairs).
Across the top of the stairs. You'll soon get use to using it. Personally I prefer the ones without a bar across the bottom - trip hazard, especially if it has to be positioned on a step or a small landing.
Even though the ones with a bar across the bottom claim to be pressure-mounted, we have always found that we had to screw them into the wall/newel post for them to work properly. (We've had three different stairgates and a room divider.)
Across the top of the stairs. We have one of those that is like a roller blind tipped on its end.
If you have a very non standard very narrow or very wide stairs finding a fixed width gate of the right dimension is quite tricky The infinitely variable width of a roller gate is a good solution. We still used ours at night until DS1 was 9 as he sleep walks.
Ours is a Kiddyguard one exactly like the one in the link.
It is impossible to not close it properly. If it is not closed properly the spring loaded roller just retracts with a very loud clicking noise which acts as an alarm in case children fiddle with it and get it open by accident. You would not sleep through it as it is very loud.
Remember you might also need one at the bottom of the stairs as well for during the day to stop DD crawling up them and then and falling backwards down them when you are not looking.
top of the stairs, definitely. And I second ABetaDad on the need for one at the bottom of the stairs... both my sons fell downstairs at about 11 months or so (not a track record to be proud of, admittedly) and on both occasions it was because someone had left the bottom gate open.