I have a Panasonic PVGS120 from several years ago. I'm looking to upgrade as well, but one of the nice features I've enjoyed is the headphone jack which allows me to more closely monitor what I'm recording. The GS400 has it, but sadly, the GS500 does not.
Here's a quote from camcorderinfo.com:
" the GS500 is only a shadow of its older brother. It is missing certain GS400 features like a headphone jack, A/V conversion and pass-through, frame mode, a 3.5 LCD, picture adjust, zebras, program automatic exposure modes, and now only has a focus ring instead of a multi-manual ring. Some of the user convenience features such as external buttons to select functions like ProCinema mode are now relegated to the menus."
Perhaps the GS400 was so full-featured that it leeched sales from the more expensive Panasonic models. Given the choice, I'd buy the GS400 before supplies dwindle.