I assume you are referring to which is the most comfortable. If so, this would depend on which airline you were travelling on, because each airline fits their aircraft out differently. For example, here in Australia I worked on 737 300's and they are extremely comfortable and popular for domestic routes because of the amount of room the passengers have between seats. But when we flew on the same type of aircraft on loan to us during the pilot dispute of 1989, there was a really obvious difference in the configuration, resulting in much less room for the passengers, and ultimately less comfort.
From what I've heard, the MD-80 is quieter than the 737's.