Leaflet focuses on being lightweight, fast, easy to use and suitable for most mapping apps, while OpenLayers is more geared towards GIS/academics with its vast built-in support for different formats and standards.

Leaflet maintainer here. AFAIK, Leaflet still has a nicer learning curve compared to OpenLayers (which doesn't affect long-term OL users like you, obviously), and a plugin ecosystem which allows for smaller library size.

The OL3 folks have been working hard on making OL3 leaner (see the "Faster, smaller, better" talk from FOSS4G Bonn here: http://video.foss4g.org/foss4g2016/videos/index.html), but I feel Leaflet still has an edge on that regard.

Also, OL3 depended on the Closure compiler for a long time, which made it difficult to interoperate with other frameworks, which is why a lot of React/Ractive/R/Django/Vue users choose Leaflet.

Other than that, there's no clear obvious advantage of a library over the other one. We have a friendly relationship with the OL3 folks, we try to copy some of their features, they try to copy some of ours, everything gets better in the end.

AFAIK, Leaflet is much easier to use. Other than that, I don't really know.