It's stated pretty well in the T&C - http://www.rockettheme.com/Terms_amp_Conditions/

But basically for $60 renewal you can use 5 templates concurrently (3 from your first year and 2 from your second),
with Pro membership you have no limit

I think the only other difference is that you get access to an extra forum section - not sure what goes on there. Will have to pay $200 to find out