I have a 50d and I truely love it. I've driven the 50i and I would compare the two like this:
50d: anywhere from rolling to let's say 140 or so the 50d feels like it has even more punch. Incredible torque and when the turbos are already moving the throttle response is incredibly instant as in a very good "normal" petrol powered car. From a standstill there is still a very little turbo lag, which you feel less in the 50i. I have now 8'500 KM on it and an average of 9.8L/100 KM, if I do a longer mainly highway trip I average at 8L/100KM. The 50i does easily 14 - 20L/100KM depending how you treat it.
50i: at higher speeds >140 KM/H sprints better I think but that is not an aspect for me as I am never there...., the definite
I gotta give the 50i for engine sound which is just beautiful. 50d in lower rpm still sounds like a diesel. If you accelerate hard it develops into a low deep growl and then it sound quit OK
Overall I find the 50d the better choice because you got all the punch you need in the speed region where it counts. Plus I did more than 1'000 KM with one tank one weekend on limited highways.