I have the Ruby 36 on my 115 gallon (5 ft long tank). As stated above, I would also suggest going larger if you have the room. I wanted to have the room for just about any skimmer, and I'm glad I got the 36 as my ATB barely fits.
Besides the red acrylic and probe holders, the difference between the Ruby 36 and the Crystal 36 is the Ruby is slighly wider and taller, which is really why I chose the Ruby.