When I was researching on which framework to replatform to, Flask seemed to have a lot of extensions (http://flask.pocoo.org/extensions/) available for it than bottle (http://bottlepy.org/docs/dev/plugins/index.html) .

Also, Flask the concept of modules/Blueprints for building larger apps. I'm not sure whether Bottle does or not, but the bottle website seems to imply that it's optimized for single file applications.

I used Bottle for a small learning project, though, and found conceptually a lot of things are very similar. We ended up using Flask because it seemed better constructs were in to use it as a foundation for a large code-base, similar to Django, but custom built with legos.

Edit: removed markdown urls