+1 for Stewart.
Although I have not personally tested the Seymour material, I can without qualification stand up for the quality of Stewart screens. In all of my years testing materials at Integrity Home Theater and at Panamorph, no brand of screen has been as consistent in terms of brightness uniformity, color reproduction, and overall build quality. Even things like the screen surround material (Velux for Stewart) have been of better quality than most competitors, absorbing light much more effectively than some other choices.
None of this is should be construed as bashing Seymour, for as I said, I have never tested one. However, going from 1.5 gain to 1.0 gain is a substantial reduction.
RE: masking. I personally find masking to be quite effective, as it makes the 16:9 image appear to "pop" more. You also get less washout from any stray reflections bouncing back to the screen. However, it's one of those things that is not, IMO, critical. If you've got an extra $4500 laying around you don't know what to do with, go for it
I don't know if you are looking at full masking from 2.35:1 all the way down to 4:3, or just a vertical mask that takes you from 2.35:1 to 16:9. I have the latter and hardly ever wish I could further mask down to 4:3. You might check into this more basic masking system (unless that is what you've already priced).
Another question - is your room going to be totally blacked out, not only in terms of light pollution but in terms of having very dark walls and ceiling? If not, the higher gain becomes even more critical as a unity gain screen will have a much bigger problem with room reflections. If you room is going to be a cave, though, a unity gain like the Seymour becomes a good option.
Have fun! Sounds like a great system.