Hard to say without knowing the models, but in general I'd say Panasonic mostly edges Samsung - but not by much.
Samsung is good for the price but Panasonic most of the time have the better overall picture I believe. My main gripe with Samsung TVs is the latest models try to remove or reduce film grain from films that are supposed to have film grain - I'm guessing it's over enthusiastic routines to help cancel noise, etc. but whatever for me this is a huge issue.
More than games (which can only look so good at 720p which most are) I want as near perfect film reproduction as possible - and a TV that thinks it knows better than Kubrick how much film grain should be in the shot is doing something wrong.
Both make very good plasma's though - no doubt about it.
A negative for Panasonic that comes to mind is I remember certain models seemed to suffer from a louder than normal buzz when in operation - maybe something to check for the model you're selecting.