As the pixel density increases the picture tends to get noisier. A better measure of quality is sensor size. More expensive cameras have larger sensors.
I think it really depends on how big you want your prints to be, if all you want is 4R prints, then most of the cams in the market are over spec anyway.
let's assume 2 cameras with the same sensor size (AP-S) but with different pixel density, i.e. one with 6.1 Mp and another with 10.2 Mp. Shooting with a lower megapixel count camera is more forgiving and you're limited by the amount of details you can blow up to. Conversely, with a higher megapixel count, you'd need to be more careful as when you pixel peep, minor issues like camera shake, focus, etc will show up clearly.
Now with 2 cameras which has the same megapixel count but different sensor size, (.e. one with AP-S and another with a full frame) The one with the full frame or bigger sensor would deal with noise better than the one with a smaller sensor.