There are a few more considerations. I think with a 3D projector solution high gain screen is important. Even in a bat cave (which I have) I appreciate high gain. The HP is a good budget solution, but ceiling mounting your projector is much more convient. I had mine table mounted for a while and worried constantly about drunks like me spilling their beers onto it. This brings me to the better high gain screen for ceiling mounts, the Vutec Silverstar. It is rated 6 gain, but really is more like 3. I love it, even in my bat cave. It is much more expensive, but you can consider this is a long term investment. You will buy many projectors, and probably just one screen. Now, considering it is a long term investment you need to think about the "Other" 3D display method, passive projection. The LG 3D projector will be using this method. All cinema 3D systems use this method. This requires a special passive polarization screen. On the other hand glasses are almost (if not) free. If you have the budget I would look into the new Silverstar 3D-P screen. If you don't, you can make your own screen frame with 3D passive material. What I found to be the best reviewed material is from a company called Harkness-Hall. I ended up buying this material myself, but haven't bothered to replace my silverstar material since I am waiting for the projector to go with it.
It has a gain of 2.8. It should work fine for either type of 3D projection. They also make full screens if you don't want to frame it yourself. They also bring another option to you. They have a micro-perf version of this material so you can have your 3D screen and rear mounted speakers. Now, myself I really don't have a problem with my speakers outside the screen area, but that is another option for you.