There's a better separation of concerns that makes it easier to customize specific pieces of the project.

For example, OpenLayers divides fetching data (Source), parsing data (Format) and interacting with data (Layer) into individual pieces. So it's relatively straightforward to handle a case where someone decided to go invent their own data format without having to subclass half the system and patch it.

For smaller projects leaflet is significantly smaller and simpler, and definitely is the right approach. But if you need to do "a lot" with a mapping system OL3 probably has it built in and supported instead of maybe available as an extension.

Thanks for the info, much appreciated! As a general aside, I feel like there would be a good audience for a blowout article comparing the top mapping library pros/cons.

That's a brilliant idea. I wish I had that article years ago. I'd even consider writing it but I've never blogged and feel it would just be a murky mess. I'd happily collaborate though. =)

OL3 can do map rotation, has Canvas and WebGL output on top of DOM like Leaflet, the projection with different CRS are better handled, more API to do geo calculation, but it has a bigger size.

Leaflet maintainer here. There is some work on a rotation code branch, plus I've been working on something called "Leaflet.GL" to do WebGL rendering. The OL3 folks just have more time and resources, you know. Also Leaflet must still work with IE8 (OL3 doesn't, there are some cases where that's needed).

We have a friendly relationship with the OL3 folks, even though sometimes we're jealous of their WebGL rendering, and sometimes they're jealous of our lower learning curve.

You can (and usually should) make your own custom build: http://openlayers.org/en/latest/doc/tutorials/custom-builds....

Even a custom OL3 build which only has the canvas renderer, a WMS layer for background and a Vector layer for POIs does not get smaller than 200k. I think leaflet is half the size.

And I find this to be even more astonishing as OL3 uses Googles Closure compiler who's very advantage should have been better JS minification (not only minimizing/uglifying but actually removing JS parts that are not needed).