Square vs Rectangle Theater

Spend the 12,500...worthwhile.

You're getting additional space for ~$65/sq. ft. That is a steal in my area.

A 16 x 28 room will be much easier to fit multiple rows. My room was 15 x 33 but I was left with 15 x 28 after I built a chamber at one end for subs. One extra foot of width would have been ideal - I could have done a bit bigger screen and moved one seat a bit more distance from the wall. You might want to take a look at how my room was arranged since it is similar to yours and I also have 3 walls with concrete on the sides. I was forced to have my entry to the theater be at the back of the room.

How tall are your ceilings? - this is another area where spending a bit of extra money might be worth it. Is this just extending the basement or does that price also extend the main floor?

You are not extending the basement! You are extending the house so the pantry and laundry area will be more functional, convenient, and provide more cabinet/storage space in the house. (Clearly you haven't been married very long nor taken a course in wife negotiations. )

There is a long and significant list of acoustical reasons; but, let's focus on wifey reasons.

- 16' wide will accommodate a nice sized screen; but, 16' of depth will not support seating distances for two rows of seats with a "nice sized screen".
- With the short depth, your back row will be against the back wall (bad for acoustics). With recliners your seating platform will force your front row to 10' away from the front wall. With a screen wall, that screen will have to be at least 2' to 3' in front of the front wall putting the screen 7' to 8' away from the front row (your screen width (max) would be 7' to 8' wide). Way too close for a big screen.
- With two rows, the room will look to be "overstuffed", cramped and not at all an inviting space to plop in for two hours.
- You'll have a big 16' wide wall with a dinky white spot in the middle called a screen.

HVAC...make certain your HVAC contractor installs both a supply and return in the room. Don't let them argue with you ... you want, will have either both a supply and return in the room or you will have a different HVAC contractor. Don't let them run duct work below the joists down the middle of the space ... it must be run next to the walls.

Avoid plumbing that will be below the joists...if there is no other choice, it should run adjacent to the walls.

The entry door should not be in the front or rear but on a side wall approximately 1/3rd the length of the wall from the front. If you have no choice but a rear entry, do not put the door in the middle of the back wall.

With only two rows, I (me personally) would not spend the money to increase the ceiling height.

Make it clear that you want the 9' ceilings to be a full 9' floor to joists. In my last house they poured the floor deep and I ended up with 7'8" instead of 8'.