They're both in the same price range and I want both. I'll eventually get both. I'm thinking the flash will be more useful until then, but I'm not sure.
I would say 100% get the flash.
it's like getting a whole new camera.
unless you are using youg camera for a lot of portraits... then you might want to get the 50mm 1.8
also, I would save for a sb-800 after you get the sb-600. the 600 would be cool for a while, but in the end, the 800 is much more powerful and has a lot more features.