Narrow bars are 'racier' and faster. But if not racing, go with the wide bars. Fortunately, I have a mix of rides and bar set-ups to compare. If on one day I ride a narrow drop and the next switch to a wide, its noticeably more comfortable with the wide, especially on climbs. Breathing is one thing but because of the extra leverage the bike seems to have a more lively yet stable handling while climbing.