Recently I noticed Costco.com that is selling both SB-600 and SB-900. However, SB-900 costs US$499 but SB-600 only costs US$224. I only have a D90 with 18-55mm VR & 55-200mm VR. I am not a pro whatsoever; so, I am asking experts from Mobile01 to provide suggestions to me.
By the way, since I am just a beginner in DSLR and I really have no idea what kind of objects I like to take photos about.