Siko, this is no doubt an exciting time for you, I just can't help but love the anticipation of getting something new, especially when we're talking about a new car!

From my understanding of things it seems that waiting for the 3er FL is not an option for you. Between the current 3er and the C, I would get a C (with an AMG pack of course). But I'm sure that you are more than aware of the risk of buying a car that has just begun its run ie quirks, defaults, longterm reliability...
But again from what I understand you are leasing the car not buying it (correct?) so those things matter less, and the C seems like a solid package from the get go.

Thats why my vote went to the C. Had it been between the C and the FL 3er, your decision would definitely harder.
Let us know what you end up getting!