Best cartop cargo carrier?

I got a nice new 2014 Forester a couple months ago, and now see I am
going to need a cargo box if I intend to carry both people and a lot of
skis. I am brand new to the cargo box and would appreciate any comments
on experiences and advice! Looks like major contenders are Skybox and Thule.

What gotchas do I need to watch out for?
Any really thin ones that might still let my car fit in the garage with
box attached?
Thanks,
gr