The only reason to get the 40 vs the 50 is to avoid upgrade-itis. The 50 has a spattering of parts which you might feel the itch to upgrade whereas if you bought the 40, it's pretty strong component package will mitigate that. I would say the 40 has a stronger component list that if you decided to upgrade the frame later on you would probably migrate, whereas the 50 you likely would just sell outright.
If you get a slamming deal on a 50, I'd take that.