these add-ons are different; collective.cover is a content type to create landing/composite pages; Mosaic is (or was) a layout solution for Plone (some people will argue it can walk over the water, but that's a matter of faith).
I would say use Mosaic:
- collective.cover is still not fully compatible with Plone 5; it kind of runs, but there are still some broken features and keeping with the updates will be painful until I learn how to deal with the static resources
- your use case seem to be layout-oriented; collective.cover works better when you have a landing page that needs to be updated dozens of times per day
good luck!