The Ertl kit definitly has more detail but I like the simpleness of the PL kit (that and I can't get a decent price on the Ertl kit).
If you are going to detail it out, there is this page on Starship that shows how to make the proton packs and rack http://www.starshipmodeler.com/r&v/kr_pax.htm
Here is a buildup of the PL kit http://www.culttvman.com/jose_fernandez__ecto-1_.html
This site has screen captures of both cars.
http://back135.tripod.com/ECTO-1Thumbnails.htm