It's not as simple as just getting the camera with the most pixels. Two specific examples are the the Canon EOS 40D (10.1 MegaPixels, APS-C) vs. 50D (15.1 MegaPixels, APS-C) and the Canon G10 (14.7 MegaPixels, 1/1.7" sensor) vs. G11 (10 MegaPixels, 1/1.7" sensor). In the case of the G10 vs. G11, Canon actually DECREASED the number of pixels in the newer model (!)
From dpreview.com's review of the Canon 50D:
"It appears that Canon has reached the limit of what is sensible, in terms of megapixels on an APS-C sensor. At a pixel density of 4.5 MP/cm² (40D: 3.1 MP/cm², 1Ds MkIII: 2.4 MP/cm²) the lens becomes the limiting factor. Even the sharpest primes at optimal apertures cannot (at least away from the center of the frame) satisfy the 15.1 megapixel sensors hunger for resolution. Considering the disadvantages that come with higher pixel densities such as diffraction issues, increased sensitivity towards camera shake, reduced dynamic range, reduced high ISO performance and the need to store, move and process larger amounts of data, one could be forgiven for coming to the conclusion that at this point the megapixel race should probably stop. One consequence of this is that the 50% increase in pixel count over the 40D results in only a marginal amount of extra detail."
http://www.dpreview.com/reviews/CanonEOS50D/page31.asp(there are image comparisons between the 40D and 50D on another page of that review).
In the case of the G11, Canon improved the image quality by DECREASING the pixel count. This decreases resolution, but increases the overall quality of the images, because it reduces the noise and diffraction issues. Another issue is the lens quality. If the lens attached to a higher megapixel camera cannot resolve the required detail, then the image quality from that camera may actually be worse than the image quality with the same lens on a lower resolution camera.
Studying the pixel DENSITY and design of sensor chip assemblies is also helpful. The size and arrangement of the microlenses on the sensor, as well as the layering architecture of the sensor makes a difference in image quality. For example:
http://www.dpreview.com/news/0908/09080601sonycmos.asp http://en.wikipedia.org/wiki/Image_sensor_formatI could go on and on, but I'll spare everybody.
The best way to decide is to actually compare images (using Photoshop or similar) from the cameras in question.