At the conferences I attend, oral presenters typically get a 15 minute slot to present their work, while posters are up for a few days, but there is a slot of around 2 hours during which presenters are asked to stand beside their posters while others are encouraged to visit (usually through provision of alcohol and canapés in the poster room during this time slot).
Most people prefer to present orally, for a few reasons: 1) Oral slots are sometimes more selective — poster slots at some conferences are for the left-overs that don’t fit into the schedule (though this is not universal); 2) Some universities will not fund scholars to attend conferences at which they are not giving an oral presentation (though this is changing); 3) An oral presentation gives you 15 minutes or more to present your work to a largish group, and those who are really interested can approach you to discuss your talk during the tea break or meal times. A poster presentation may see your time monopolized by one person, while others struggle to see past them to your poster, or it may mean a lonely two hours standing by your poster while others enjoy the canapés and socialize.
Some presenters, though, prefer the more intimate one-on-one conversations that poster sessions enable, or simply don’t like speaking in public.