You can make the calculation when we know the numbers of safecoin you will gain per X. I now some here have already bought Odroid or other devices so you might get some reviews after they tested them later.
I have no idea! Even when we have data it will be hard to compare similar devices that are not running side by side with identical connectivity, OS and identical vault configurations.
It’s one reason I bought two U3 was with the idea of trying different configurations (eg vault sizes) side by side on the same internet connection.