If the up-charge of buying the diesel isn't an issue buy the diesel. If you prefer to save the cash get the gasser. Both will do the job. The diesel will leave plenty of room to upgrade your TT, TH, or 5er later.
I purchased my first diesel this summer. I traded a 6.0 liter GMC with 4.10 gears and couldn't be happier with performance and fuel economy of the Diesel. The fuel economy is saving me money over what I was spending with the gasser loaded and empty.
I have no idea if it will be a good investment long term. That's not why I bought it. From what I saw while shopping for a new TV the diesels definitely retain more value as the odometer reading goes up.