Well, the 46BF has an advertised size of 36x15x20. Not sure if the 15 is the sides or the middle. The 55 (AGA brand) has an advertised size of 48x13x20. I'm guessing the surface area would be very close with a slight advantage of width with the 46BF. A 55 has 576" sq surface area. A 75 has a surface area of 864 square inches...a significant improvement IMO
...and it's still only a 48" tank. I'd look at a 75 very hard before I bought...it sure would give you more options later on. The difference in wholesale prices according to my catalog is about $25.00 for regular black tanks. Something to think about anyway.