The Slaughter Dome just sounds like a new version of the really shitty Mad Moxxi DLC from Borderlands 1, except this time you can actually level up from it.
Does GameStop accept international credit cards? And also, Creature Slaughter Dome vs. the $10 credit. Comments?
That or just waiting for the eventual $39.99 sale :o
My guess is that the Slaughter Dome will be available to download later for $10 (but I wouldn't be surprised if they charged $15). So really it's up to you to figure out which is more valuable, DLC or credit.