Enma is not a better sword than Shusui, but it has a high learning curve. Zoro can cut that cliff with any sword but Enma has a condition where it draws out too much haki, which means that Zoro cuts that which he does not mean to cut. This, as we know, is not a desireable trait for swordsman. A true swordsman only cuts that which he intends to cut.
So in order to master Enma, Zoro must improve his haki control, and that's why he said that with Enma he would grow stronger(since better haki control= better swordsmanship).
So in order to master Enma, Zoro must improve his haki control, and that's why he said that with Enma he would grow stronger(since better haki control= better swordsmanship).