If by ace you mean best pitcher in our rotation, then Buehrle is our ace. If by ace you mean a dominating shutdown pitcher, then Buehrle is NOT an ace.
Johan Santana is a dominating shutdown pitcher. Buehrle gives up far too many hits, doesn't strike out enough batters, and depends far too much on the defense playing behind him. Buehrle is a great pitcher and a massive workhorse but he definately isn't in Santana's league.