Honestly... I am in the same shoes as you....
been debating on getting a flash or lens..... so far, I bought a new lens. Now, I am debating, if I get another lens or flash. I am thinking of getthing a 430ex II or the 580 ex II(2x price of a 430ex II)... but I think I will have my answer after a studio photo shoot on Friday...meeting with a stroby.... GGGGAAAAASSSSS.