I think that, for users, the OPAC will have the greatest impact on library users, especially in large and academic libraries. This is because information that previously required a specialist to find is now much more readily available, and because catalogs are becoming more and more faceted, they are also becoming more and more easy to search for the average user. Having information more easily accessible, especially with regards to specialized subjects, means that users no longer rely solely on library staff or browsing to find things.
On the other hand, ILS is, in my opinion, the most interesting development for librarians. The simple fact that it updates the same information across databases and keeps information consistent and accessible (most of the time) is, as one would expect, important for an information professional. In addition, the OPAC would not be possible without the data fed into it by the ILS.