Ok if we're talking about one game I take Pedro even with Roger's 20 K games. Let's take a look at Roger's two 20 K games and then look at Pedro's best game ever.
First one against the Mariners in 1986, If you look at that Mariners team they lived at the bottom of the standings all year and turned out to be one of the worst teams in the history of the game as far as strike outs go.
The second one against Detroit also a meaningless game against a bottom of the standings team.
Now lets look at Pedro's best single game performance, he went into Yankee Stadium: one of the most hostile environments to play in, in September during a pennant race: one of the biggest games of the year. He struck out 17 and gave up one hit. Probabaly one of the most dominant pitching performances in baseball history. He got a standing ovation in Yankee Stadium by Yankee fans, probabaly the only time that's ever happened to a Red Sox player.
If it's one game there's no question I take Pedro, anything beyond that I take Clemens.