Both fish have issues to be concerned with. The Powder Blue tangs usually comes into LFS in bad shape and a lot die within a short time. They're also ich magnets and are easily stress out, which can lead to ich or them dying. They're also aggressive and can cause problems in your tank not only with other tangs but with other fish. A lot of them die even if they're eating. Care should be taken when purchasing one, making sure it's eating and showing no signs of stress or irregular swimming habits. They should also be quaratined before placing one into your tank.
Powder Brown tangs usally comes into LFS in bad condition. On top of that because they're very skiddish they usually don't do well in LFS tanks and usually come down with ich or are really stressed out. They do get large and need a large tank with plenty of swimming space to do well.
