While IAD might be a less busy airport/terminal and be a bit better organised, I think that for those with lounge access JFK wins overwhelmingly. A few salient points, IMHO
1) the quality of pre-flight dining in Club is much better at JFK
2) I've not experienced the Concorde lounge at JFK, but at IAD its really not good. Poor food, poor quality drinks at IAD.
3) No first/gold lounge at IAD, for gold card holders its the galleries club lounge.
4) The galleries club lounge at IAD isn't great - really poor quality wines, corridor layout, serviced bar instead of self-serve bar.
5) For non-lounge passengers, IAD is much less crowded than JFK, so better on that point.