expansum and pileatum are "somehow similar", however, the labellum of pileatum is a bit rounder and somehow more elegant.. of course, this is very subjective, so let's go for the diagnostic differences.
both pileatum and expansum have a more or less marcant depression in the "middle" of the labellum, however, expansum has always a very distinctive callus in front of it
Catasetum expansum 2010-03-14 01 | Flickr - Photo Sharing!
Catasetum pileatum 'Christina' | Flickr - Photo Sharing!
Also, the labellum of expansum tends to reflex strongly while pileatum does not (Note: Recently I have had a discussion with a friend who showed me a photo of a pileatum from south of Rio Negro, which aparently can have reflexed labellum too. He mentioned that apparently this is not uncommon in this population... I only know that photo!)
both species can have a very wide range of colors, being the tipo of pileatum white and the tipo of expansum greenish.. but you can find both species in yellow, green, white, red, bicolored, with spots... note: the spots in expansum are normally large and "bulky", while in pileatum they are very small and "never" bulky (note that I have written
"never
"... this is my understanding of this species, but cannot discard the possibility that at least one plant has bulky spots - there is a lot of genetic introgression from macrocarpum and possibly other species into pileatum!)