like it has been said what are your needs?
Do you have another, family car ......
if this would be your car, that occasionally transports the family, then I would go with the 640D, purely cos its different.
Personally I think it weird that BMW called the car a 640D when in fact the engine is the exact same as the 535D and the 335D (both of which are not 3.5 litre either)....