rectangular trampolines
In Sundance's opinion, the rectangle is the safest design because of the physics involved and is the only design used in trampoline competition. The springs criss-cross each other to create an even bounce over most of the trampoline mat's surface. There is little pressure to throw a person off-balance when they move away from the center. The bounce is much softer, better and more controlled because the springs in the center of the rails stretch more than the springs at the end of the rails resulting in more vertical travel.
The rectangular design requires much heavier and stronger materials because the stress is mostly on the center portion of the rails, therefore the greater cost to build. However, since it is built much stronger, the life of a rectangular trampoline is much longer. Note that circular trampolines are never used in trampoline competitions.