While Sundance carries all shapes and sizes, including round, 12-sided, octagon, elongated octagon, square and rectangular trampolines, safety and quality factors vary with each shape.
Because stress on the round frame is equally distributed, lighter, less-expensive frame materials can be used.
By comparison, the rectangular design has its stress focused mostly on the center portion of the rails, requiring much heavier and stronger materials. However, not only does the rectangular last longer due to its strong build, it has the best geometry for trampoline safety.
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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.
Octagon & 12-sided Trampolines
The octagon & 12 sided trampolines are much safer than the circular design. The springs on an octagon offset the center because of the eight straight rails, so they end up criss-crossing each other to some extent. This results in less pressure behind you and you are less likely to be thrown forward off balance and into one another. There also is more vertical travel in the mat because of eight straight sides. The reason for this is because the springs in the center of the rails stretch more than the springs toward the end of the rails. This produces a softer, better and more controlled bounce then the round design.
Square Trampolines
The springs on a square trampoline criss-cross in similar fashion as the rectangular, but there is not as much vertical movement in the mat. The result is a much harsher bounce because of the further distance from the springs. The further a person is from the springs, the less travel there is in the mat.
The quality of the bounce is similar to that of the circular design.
Round trampolines
The round trampoline is the least safe shape. The reason is because of physics. In a round trampoline every spring points right into the center and makes the center soft. When you move out from the center, the bounce gets harder behind you. If you lose your balance or concentration you may be thrown forward off balance. Fifty percent of trampoline injuries occur when there is more than one person jumping, and on a round trampoline the risk of collision increases. Bouncing on a round trampoline is like jumping into a 14-foot funnel–everyone will go down the spout.
However, because the stress on the frame is equally distributed, a lighter, more cost-effective frame can be used and so round designs are often attractively priced. But don't let anyone tell you that they are safer because they keep kids in the center.
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 octagon & 12 sided trampolines are much safer than the circular design. The springs on an octagon offset the center because of the eight straight rails, so they end up criss-crossing each other to some extent. This results in less pressure behind you and you are less likely to be thrown forward off balance and into one another. There also is more vertical travel in the mat because of eight straight sides. The reason for this is because the springs in the center of the rails stretch more than the springs toward the end of the rails. This produces a softer, better and more controlled bounce then the round design.
The springs on a square trampoline criss-cross in similar fashion as the rectangular, but there is not as much vertical movement in the mat. The result is a much harsher bounce because of the further distance from the springs. The further a person is from the springs, the less travel there is in the mat. 
The round trampoline is the least safe shape. The reason is because of physics. In a round trampoline every spring points right into the center and makes the center soft. When you move out from the center, the bounce gets harder behind you. If you lose your balance or concentration you may be thrown forward off balance. Fifty percent of trampoline injuries occur when there is more than one person jumping, and on a round trampoline the risk of collision increases. Bouncing on a round trampoline is like jumping into a 14-foot funnel–everyone will go down the spout. 
