User blog comment:Lykrast/CCC4 by Commander Marko/@comment-5961106-20140709094917/@comment-1330314-20140709095412

The first wording is a lot more complicated, whereas the second is more direct, so I'd go for the second option. While just a technicality, the first wording (10 second base cooldown, cooldown reduced by 5 (- 1 per stack)) would also make the ability go outside the bounds set by the competition (basic abilities cannot have a base cooldown over 5 seconds).