I would agree with Clown-lover.
I have a 100 gallon, 5 ft. tank and I have two tangs, a yellow and a hippo, and I struggled with deciding to keep them in a 5ft. They swim all over the place, and my hippo especially will dart from one end of the tank to the other as if she is in a race.
125 gallon is a nice size, but I have often read that it is the horizontal length that is most important when deciding to keep tangs.
If you are torn as to what type of Tang to take in, I would consider a yellow personally, they tend to be more graceful not quite as race horse type of swimmers compared to the hippo and powder blues, at least in my experience anyways.