That's the same Q I asked myself when buying and alfa 147 and this is what I told myself.
1. Do I want the car? Yes
2. Would I like to drive another alfa after this car and stick with the alfa brand? most certainly
The thing with resale value is: are you are going to trade it in on another vehicle of the same brand? If you are, then you will obviously get a better resale on your current alfa, at alfa, for a new alfa. But if you take the car into renault, all they will do is phone alfa to sell the car, alfa will give them a **** price (because they want you to buy another alfa) and you will get a crap resale value.
So i'm sticking with alfa who I know will give me a good trade in if they want me to buy a new one.
Well, that's how it works here in South Africa. May be a little different where you are and it probably only pertains to purchasing new vehicles.