I agree with you. People don't want to get Pearl because it's pink! Personally, I don't care what color the case is. Plus the people who get Pearl are getting a better deal because the pokemon in Pearl are A LOT MORE USEFUL THAN IN DIAMOND. I also think that Palkia looks WAY better than Dialga. I think it's kind of ugly.
You know whats funny? In a couple months they will probably make a game that incorporates both diamond and pearl. My evidence? Yellow version (red and blue), Crystal version (gold and silver), Emerald Version (Ruby and Sapphire). I bet money that they will. So then picking wouldnt matter. Its bound to happen
Based on those I talk to, people who play Pokemon seriously seem to favor Palkia. I'd say get Pearl because yours will be more rare if Diamond sells better, but thousands of people are cloning the rare pokes from both games already, so it won't really matter.