Whenever anyone asks me about megapixels, I respond with another question: How big of prints do you make?
In this case, if you plan on making enlargements bigger than 8" x 10" with your pocket digicam, then I would say go with the Sony. 5 megapixels will get you up to 11x14.
But if most of your work remains on the computer, or is 8 x 10 or smaller, I really like the Contax. They both have Zeiss zooms, true, but the electronics of the SL300RT* are actually tuned to the lens. When you shoot at ISO 100, this is apparent, at least to my eyes.
Both the Sony and the Contax are very good cameras. One uses SD, the other Memory Stick. Figure out what you want out of the purchase, print wise, and I think that's your answer.