If I were to buy another, it would be a 4450.  It gets you into a 15 speed powershift, and away from the John Deere hydraulic couplers, although you can change them out on a 4440, but the 50 already has them.  Other than a bit updated interior, its basically a 4440.  I have a 4440, and we did have a 4450.  Both had quad range so that's why we kept the 4440.  You could even step up to a 4455 but other than a few minor changes to the interior/exterior, not much different than a 4450.