Charlie (from Novak) has stated several times than the GTB is a better brushed
speed control than the GTX, along with having the ability to do brushless. It's got a lower "on resistance," and whatever special things make brushed ESC's do their thing.
If you search the Novak Brushless thread for Charlie's posts, you'll find where he's said this himself. So, if you don't mind the size, it seems like the way to go, especially if brushless is in your future.
Edit: Here is a link
to the post where Charlie states that in brushed mode, the GTB is the most powerful brushed ESC ever.