Oak and Birch each have great cosmetic qualities, but their use depends on the specific situation, such as surrounding conditions. Any pics of the room, the tank, stand being covered, etc.? Generally however, Birch is going to show a very fine grain and be lighter in color/tone. I'm not a big fan of Birch because it's often used/associated with cheap veneers and coverings. Oak is a harder look, will stain darker and have more of a defined character grain and look.