i've honestly no idea regarding slicks, but having used both these tyres in treaded from there's a subtle difference in the way they feel. The Metzlers seem marginally more pliable and forgiving on rough surfaces and when they let go there's a little bit more shimmy & warning. The Pirellis feel slightly harsher & seem better suited to smoother surfaces.
Both are tragically vile in the cold & wet though! These days i use Dunlop D211's which seem to grip damn well regardless of road surface and can cope with adverse weather conditions better.
But that's all about treaded tyres - no idea about slicks -terribly sorry!