On piano, Eb, Ab, and Bb, and F# are quite more common key signatures for pieces than their counterparts of D#, G#, A#, and Gb. The C#, however, is usually referred to as Db (we're talking about major keys here... C# minor is perfectly fine).
This has to do with ease of thinking. On guitar, each of the notes are one fret apart, there's no need to think of black/white notes. On piano, it can be confusing to think of hitting that B# if you're playing in A# major. When one sees a B note on the staff, instinct leads the player to either B or Bb depending on the key signature. Seeing a B# is confusing because you're playing what LOOKS like a C. It screws up the fingering and is just a mess!
There is no denying, though, that an Eb is enharmonic to a D#... anyone with piano lessons (or music lessons of any kind) should know that. Hope that helps answer your question.