How simple is mapping portion of the app you are building?
The simpler the mapping requirements, the more we lean toward using Leaflet. Leaflet does a fantastic job of handling the most common mapping tasks like consuming base map tiles, panning, and zooming, and its API is easy to understand and simple to use. Once you go beyond the common tasks, however, you need to use Leaflet Plugins and this where things become more complicated. There might be functionality that you are looking for that unfortunately doesn’t exist.
If you have a feature rich application with lots of functionality, we gravitate toward using OpenLayers 3. OpenLayers 3 has a larger and more complex API that will allow you to do all of the common mapping functions that Leaflet does plus many more. For example, if your application has requirements involving map projection, OpenLayers 3 will allow you to accomplish these requirements more easily than if you are using Leaflet. Of course, with a more robust API comes a steeper learning curve, so keep that in mind when providing estimates to your project stakeholders.