If you are really sure its gonna just be fowlr
I would go with a 55g. The extra few inches will be needed for those type of fish.
The 40b is a great choice for a reef, I just set mine up and it is pretty awesome. I drilled my tank with bits I bought on amazon.com. To be honest, my reef club president did the drilling and it seemed sooo simple, just go slow.
For powerheads though, I would go with 2. 850's if you go with the 55 for sure and 550's if you go with the 40b. I have an mp10 in my 40b and its not enough flow. :-( I threw in a 425 for now but will have to get another higher gph
pump as well.