Larry noted an interesting passage in "We Were Young and Carefree" about Fignon's 1989 season. Fignon writes: "There was a new technical development available to us that year. At the System U team we had begun to use Michelin high-pressure covers (as in clinchers, Larry says) It was a kind of revolution. Tradition demanded that the thinnest possible tubular tires were used by the pros: 20 mm in general. And now, not only were we being given standard covers to ride, but Michelin asked us to use 23 mm section tires. Three millimeters sounds like nothing at all but it seemed impossible to us. We were not proud of them, and we didn't have confidence in them. Of course, the most advanced technicians from Michelin came to present the data on the issue. They were determined to prove to us that the amount of rubber that came into contact with the road didn't change even if the tire was a little wider: it would always be 8-9 mm. For them, the big change was elsewhere: how the tire reacted when the bike was cornering. They assured us there would be more tire in contact with the tarmac so it would hold the road better. During our first training sessions on the tires the problem was in our heads rather than anywhere else. The size of the tire seemed 'large' so we felt they were slowing us down. Then we raced the Tour du Haut Var. It was the best possible weather for testing the tires: it lashed down with rain all day. And miracle of miracles, on the descents, using the tires we were leaving everyone else behind. No one could keep up, they couldn't even hold our back wheels. It was fantastic, the roadholding was exceptional and here was a considerable technical advance. As far as rolling resistance went the difference was actually minimal and they were definitely more stable."
Very interesting comments from a guy who, at the time he wrote this, had nothing to gain by extolling the virtues of these tires. Heather was sponsored by a team supplied by Michelin a few years before this -- the riders on this team thought the same thing in the beginning and only after the team boss forbade them racing on anything else did all of them start to experience the characteristics of these tires. Michelin also sponsored the Carrera Team of Claudio Chiappucci and the Gatorade Team of Gianni Bugno. We witnessed the wet weather cornering superiority during a wet Giro stage as "El Diablo" repeatedly dropped his companions on a twisty descent,just as Fignon described. We also spoke to a Gatorade mechanic to find out if they really were using these tires rather than some tubulars badged as Michelin clinchers -- we met Marco at the team truck and saw him mounting the tires - clincher rims, inner tubes, NO GLUE! He, as a mechanic was especially enthusiastic since he could just install the tires, no mess and no waiting for glue to dry before the wheels could be used. It truly was a revolution - one that Larry was slower to adopt than Heather, sticking with "tried-and-true" tubulars until the cost of decent-quality examples became onerous. Once he switched he never went back, after finding the performance claims to be true. A few years later we spoke to a Michelin rep who told us of how they, who also owned the Wolber tubular tire concern, would quietly supply Banesto's "BigMig" with clincher wheels for the big TdF mountain stages, where his tubulars would shift on the rim as the glue softened under braking heat, leaving an annoying lump at the valve stem. On these most-crucial stages with the harrowing descents, Mig trusted these CLINCHERS rather than the old-school tubulars!
You might wonder about the current pros -- why have they gone back to tubulars if clinchers are so good these days? Think about these facts -- with the clincher now accepted by most cyclists, there's no reason for the makers to pay teams to use them as they did back in the late '80's or early '90's. Second, tubular wheels are still lighter, especially when made from carbon fiber, a material which does not work very well in a clincher shape. Third, all riders fear a flat tire coming off the rim at high speed. While this is extremely rare, a tubular, since it's glued onto the rim, has far less chance of coming off.
If, as a pro team boss you get whatever tires you want at no charge as well as the wheels while no clincher company is paying out more sponsorship monies than the tubular folks, plus YOU don't have to glue on the tires yourself (someone else gets to deal with that mess) and you'd prefer not to worry at all about losing a star rider for the season due to a crash from a tire coming off as a result of a flat (though tubulars roll off the rim from high cornering loads while fully inflated now and then) I think you'd pick the tubulars too.
If none of this applies to you, I think you'd choose the clinchers, as the vast majority of us (even the stubborn traditionalists like Larry) do these days.
(FTC disclaimer: Heather likes Michelins, Larry likes Vittorias and we both use Torelli's "open tubulars" (clinchers) as well as they are an official supplier to CycleItalia.