I've seen the 37" plasma and the LZD80 up close at a really good dealer who was able to swap between SD and HD material. I would say that the LCD does a pretty good job with SD material compared to many TVs of this type, but the plasma reveals more subtle detail and definition (faces are quite a good measure) and produces a sharper, more pleasing picture overall.
With HD material, the LCD set has a definite and seductive advantage - its full HD image is strikingly bright and clear. The plasma is less sharp but still very good, colours are more subtle, the black level is better - maybe the picture is more 'natural' despite being less startling.
If we were able to watch everything in HD, I think I might be swayed by the LCD set but, for watching a lot of day to day TV plus highlights such as HD sport/Blu Ray movies, the plasma is IMHO a better compromise.