I personally would go with the 11 X 20 room. Acoustically treat that well so that you can do vocals in same room without wasting money on an isolation booth for vocals. Every singer, rapper, yelling guy in my studio has loved the fact that they are not isolated. The only negative is that I personally hate headphones. But that is essential when you record in the main room.
So, what I have done in my control room is open ceiling with pink fluffy stuff. As advised by JH Brandt. It is covered in cloth to just behind my listening position. I have 6" thick rockwool panels behind the desk in corners from floor to ceiling. I also have a 4" thick 2'x4' panel 'cloud' above me at listening position. When mixing, I bring in two panels from my drum room at reflection points on side walls at the reflection point from monitors.
In the rest of my 13x30' room I have 4" 2'x4' panels hanging on walls with 1.5' gaps in between. So 8 of those panels down the walls at the long walls. I haven't had reason to place/build bass traps at the end of the room as it defeats my purpose for the room. At that end is a $2500 watt PA system that some live guys love to hear when drums are recorded in it's 13 x 30" isolation room. In which the ceiling is completely filled with 4" rockwool with 6" gap between the insulation and subfloor above. Again covered with cloth. Then 6 of the 4" 2 x 4' rockwool panels staggered on the concrete walls. There are also a few hanging office dividers that I have at angles to reflect the sides. It is a haphazard thing that just worked really well. Though the drum room is a different animal, and treating it has much to do with trial and error. This just worked well for me.