In all the aspects, enhancements are more powerful than modifications, even given all the modification support that transaction SPAU offers. The reason is due to an important difference between an enhancement and a modification that stems from the conceptual difference between the two technologies.
In contrast to enhancements in the Enhancement Framework, modifications are physically part of the object they modify. This means that every single modification gets lost in an upgrade and needs to be re-inserted even in cases where the underlying SAP object has not changed at all!
SAP NetWeaver 7.0 (formerly 2004s) offers the new Enhancement Framework that not only is intended to unify the modification and classic enhancement techniques, but also offers you almost the same flexibility as modifications without the limitations of modifications.