To take off on your truck analagy I think a good comparison here might be suggesting to someone who drives in the city daily and primarily uses their truck to carry groceries buy a 3500 dually because they might tow something someday. They could buy an F-150 for 30k or the dually diesel for 55K. Which is better? Or which is better for THAT driver?
I do agree that it is important to buy a body that is comfortable and has a user friendly interface. You have indicated several times that you like the technical edge of the D300 so that gives the impression that was a primary factor in your choice. In a P&S style shooting you won't be utilizing any of that ability any more than the person using the 3500HD for shopping. I'm sure you will be thinking.....Yes but I can grow into it. The caveat there is *will* YOU learn to use it and utilize it before that technology becomes dated. The tecno twist can really get you you wound up and spending a lot of cash that is not necessary. While you may in fact out grow a P&S I think it unlikly you would out grow any of the bodies listed by the OP. Canon still has the best range of high end glass. Most noteably primes. However Nikon is really stepping up their game. Competition for the consumer is a beatifull thing. In the next few years we will see even better bodies and lower prices but I personally hope they stop the MP wars. The last thing I need is larger files draining my HD.