They all run quiet and cool.
Its what you do which makes it runs hotter and louder.
Generally, the Quad Core will run hotter than the dual cores. But Apple manages to keep it cool when idle and doing not intensive tasks. It will become hotter when: Flash is playing, video is playing, gaming, opening many apps at a time, many tabs in web browser, and many other reasons.
the last gen 13" would run cooler than the 2011 13" but would be less powerful.
Heat is not a problem to worry at all. People who form heat threads are just damnnnn to sensitive to heat.