I'd go with the 120 as well. I currently have a 55, and I'm currently trying to find a nice used 120 for cheap, since they really don't take up that much more room, and give you more options for fish. I feel that my 55 is too small for my 5" purple tang, plus I love the depth of a 120. But now after reading Sniggir's reply on this thread, saying that there is such thing as a 150 gallon tank that is 5x2x2, I'm going to be looking for one of those, because I have 5 feet of room for the tank, but not 6... Anyways, yeah... get the 120... If you get the 90, you'll probably just regret it in the end.