- Leaflet is quite easy to use, yet powerful enough for many if not most applications.
- OpenLayers 3 has more features but requires you to learn more before you can use it.
OpenLayers is used at our company to provide an application that is used in a number of locations, namely for the map of Cologne (at www.cologne.de). Seems as if they don't update their English pages that regularly, koeln.de is updated considerably more frequently.
Anyway, for a private project I use Leaflet instead. It visualizes the location of the Stolpersteins in Bonn and the nearby region. You can find it at stolpersteine.codeforbonn.de. It is part of the Code for Bonn project which in turn is part of the Code for Germany project, the German twin of Code for America.
The stolpersteins shown are taken directly from the OpenStreetMap database so adding a new stolperstein to the database means improving OpenStreetMap.