If you’d asked this question not so long ago, the resounding answer would have been tubulars. However, today’s clincher set-ups are arguably the faster option.
If you first consider rolling resistance, the rubber compounds used for clinchers are no longer inferior and, if you use a lightweight latex inner, that’s what you’ll find sewn into a tubular so, no difference there. Also, tests have shown that using tape or poorly applied glue can significantly add to a tubular’s rolling resistance.
From an aero point of view, with modern wide rims and correctly chosen tyres, the aero profile of a clincher is far superior to a tubular. However, pick the wrong tyre and you can easily negate those gains.
Weight wise, a like-for-like tubular set-up will always be lighter but, unless it’s an uphill-only event or you’re racing a crit with hard accelerations, aero trumps weight and the clinchers will be faster. You’ve also got to factor in convenience and there’s no doubt that living with clinchers is a lot easier and more cost effective than tubs.
So, why does the pro peloton still favour tubulars? Partly due to tradition and partly because they have team cars following loaded with spare wheels. But the main reason is that you can keep riding on a soft or, in extreme cases, flat tubular which, in a race situation, can be crucial.