Upgrade the projector for sure! The two screens you mentioned are pretty much almost the exact same thing. Both white with 1.3 gain. Both are very nice screens. You are not going to get any difference with a new screen unless you drastically change the material (i.e. grey/black toned or super high gain). Since you have a completely dark room, you're best result will actually be with the standard 1.3 gain screen. Put your money into a much better projector.
You can also look at Sony projectors. The JVC DLA projectors are noted to have very excellent black level and a film-like quality. The Sony projectors are excellent as they make the image "pop" a lot more. The Sony is a little bit more exciting but does not do as good in the lower blacks (shadow detail).
You can also look into a Lumagen Radiance video processor and then do a calibration.