What is the difference between 30a and 10a solar controller

Assuming the 10A controller is appropriate for the setup being sold and these are both for the same voltage batteries, the main advantage of the larger controller would be that you could add additional panels in the future without having to upgrade the controller. One or the other might also be a better/nicer/more efficient controller, but of course there's no way to tell if that's true or not without further details.