Don't fall for the "more pixels = better image quality" myth. There is a tradeoff between high resolution and better low-light performance. Unless you plan to make drastic crops or are in the habit of making big (16 x 20 inch or bigger) enlargements, you probably won't notice the resolution difference between 10 and 15 megapixels. However, as pixel count goes up, the active area - and light sensitivity of each pixel - goes down. You therefore start picking up pixel noise and losing dynamic range - the ability to hold detail in the brightest and darkest parts of your image simultaneously.
Just to add to that:
This is why top-of-the-range DSLRs employ a full-frame sensor. You get more pixels without the resultant drop in image quality. But the downside of this is that you pay a heck of a lot more for a lens to focus light to cover the full surface.
True enough. You also pay a lot more for the camera body itself, but I don't think all of that extra price has to do with the bigger imager. These are usually the company's top-of-the-line flagship models that have all the bells and whistles and have sturdier construction. The buyers of these cameras also tend to be less cost-sensitive than the purchasers of the lower-end models. The companies can therefore get away with charging more than they otherwise could.