I have no direct experience with the Goo Systems products, but the reviews online seem favorable at least as far as it being a good value and a cost effective solution, it definitely would likely be a significant improvement over a typical wall. There are numerous references to how since it is a paint product the surface preparation is critical and any imperfections in the wall surface will show up in the image, it makes sense that this may be even more of an issue with larger screens areas. I also noted that most of the photos that Goo shows in their gallery are of applications where there is low level, controlled lighting, the few with some ambient lighting on the screen seemed to show quite a bit of wash. Unfortunately, some of the pictures on the Goo Systems USA site also appear to possibly have some PhotoShop involvement, such as lacking any fringing around an image that does not fill the screen and in some cases almost a different resolution for the image than the rest of the picture. I haven't seen anything addressing this aspect, but since the Goo Systems is a multi-coat system of two different products I am curious as to how easy it is to touch-up or repair.
I have several experiences with the WallTalker products. They pretty much do as advertised and are fairly easy to install. Like any multipurpose product, in this case being both a projection and writing surface, there have to be compromises and the performance for projection will not be the same as with a dedicated projection surface. It is also a difficult surface to work with if you ever need to remove or repair it, think very heavy wallpaper that is strongly glued to the wall.
You didn't say what size image you are considering, but I look at getting prices for a couple of different options. If nothing else it may help show what a good steward you are being with the church's money.