This is a tough decision...I went for the lens first, only because I managed to "borrow" a flash for use in the meantime before I got my own Speedlite.
If you already have a lens and is comfortable using it, then the flash is the next-in-line purchase. Otherwise, I think the lens is more important than the flash...without a good lens, even the best Speedlight is of no use. :think:
I suggest to hold on a second with your purchase. With your question, I worry that you don't know what you are trying to get at
I believe you already have a lens (because only a super duper jerk asking which one to get, if he only have a camera body). So make use the best of it. Over the time you will realize what you are missing. By then, you don't even need to ask that question
Then you'll need a lens (better yet, a macro lens) first. You still can get very good macro shots without using a flash when the light condition is okay, but you might not be able to get very good macro shots with the kit lens.
A Sigma 50mm f2.8 macro lens costs less than $350 only. Can get better ones if you wanna put more money in