[Pinewood]

I tried both LCI diesel versions before buying the x30d.
It was a lot better in midrange acceleration and sounded better under load. The bigger lump is quieter as it doesn't need the higher revs to deliver the power, and when it does, the sound is a very pleasing growl. The 20d was more a whine. The x20d needed more revs to get the best from the engine, whereas the bigger lump needs very little input to start feeding in the torque. It was more linear.
In everyday town/commute driving, there isn't a whole lot between them, but once you demand more from the X3 in terms of towing or carrying 4 people + luggage, or some spirited fun driving - then the 30d gives you more of that X factor.
Fuel economy wasn't a major concern for me, and in real world consumption i get 36mpg with a mixture of city (central london commuting) , and semi rural driving.
The x20d is adequate for just getting about, but the 3.0 engine turns the X3 into a formidable vehicle.
Lastly, the subjective part of the equation for me is "knowing" i have the power/acceleration on tap at all times. This is going to be my car for the next 7-10 years, so i wanted the better performing engine.

As above - get the bigger engine, you won't regret it. I don't see many X3's on the road, and when i do, they invariably are the 20d versions.