To determine which seating arrangement would be able to seat more people you will find the distance around each table.
For the rectangle, you will find the perimeter:
P = 2l + 2w
2 x 6 + 2 x 4
P = 20 feet
For the circle you will find the circumference:
C = pi x d
3.14 x 6
C = 18.84 feet
The rectangle would seat more people because the distance around the table is longer.
You would be able to fit about 7-8 people around the rectangular table and 6-7 people around the circular table.