Some people who have diplomacy and then immediately got Rebellion felt it was a bit too pricey for what it added, considering it was a stand alone expansion (and thus intended as a great deal for new players), even with the $10 discount for previous owners. So if money is a big thing for you, I'd say you can certainly wait for the price to fall a bit with this one. What it adds is good, but its only going to get better and more balanced with time, and more mods will be available later. So as long as you don't forget and their is another game you really want now, its probably a safe bet to wait.
