I have almost exactly the same setup - currently building a HT that is 20' wide, 24' long. I will have two rows of seats (4 per row) planned, one at 14' and the other on a 12" riser at 19'. I am definitely going with a 2:35 aspect ratio screen for two reasons - first, we will mainly use the HT for late release blue ray movie watching with minimal use for anything else; second, based on installing a 138" diagonal screen, a 16:9 screen will challenge my back row viewers with the lower screen area. I want a true cinema type screen, and it will also be a curved screen to minimize pin- cushioning. My front wall area is 19' wide and only 8' floor to ceiling due to extensive ceiling design work.
If you are really a movie buff like me, you might want to consider a 2:35 and/or curved screen.