Sensor in the S95 is the 1/1.7" type, 7.6mm x 5.7mm. Sensor in the LX5 is 1/1.63", 7.89mm x 5.81mm; but with its variable aspect ratio, you never use every pixel of the sensor. Regardless, difference in size is negligible.

The larger size of the LX5 allows a lens with larger aperture in the long end, and just wider in general. The LX5 is 24-90 (35mm eq.), F2-3.3. The S95 is 28-105 (35mm eq), F2-4.9. While both are F2 at the widest, the lens of the S95 stops down fairly quickly. At 85mm, the S95's largest aperture is F4.5, which is almost one-stop slower than the LX5. And of course, it's not as wide.

I have the S95, as its small size is more important to me.