The guys over at reddit.com/r/trampoline say that a rectangular trampoline has a larger sweet spot AND provides more air. Typical dimensions are 9x15 10x16 to 10x18
I can see the following cases:
- Consider a line running down the middle of the long dimension. Suppose the trampoline is 2 W wide by W long. Once you get within W of the end, you start having edge effects, maybe a bit further from the end. This gives you a long narrow strip of equal response. Since many bounces are either flat or sitting, this gives all parts of your body even treatment.
By contrast a round trampoline will be 'softer' in the centre than around it.
- A large round trampoline requires that more air be moved with the mat motions. This energy loss is parasitic.
The argument made on one forum is the rectangular matt has more springs. This seems specious. If it were that simple, they could use stronger springs, or space them closer together.
Comment asks what would equivalent sizes be? Hard to tell. In effect the outer foot to 1.5 feet are unusable. In round sizes the range is from 10-16 feet, with 13-15 being most common, although there are a lot of 11's. 12 isn't common.
In rectangular the minimum I've seen is 4x9 which gives only a 2x7 usable area. In squares 11x11 and 13x13 are common. But the guys who are doing athletics say that a 10x16 or a 9x14 works better than a square one.
Both square and round ones tend to have an radial component to the rebound inward, the round one more so than the square. This is touted as a safety feature, as you have to be more "off target" on your landing to land on the frame on your next bounce.
Comment asks what an athlete consdiers important:
A: bounce. How high can he get with good control. I've seen some matts that are actually nets with 1/8" cord and about 1/2" spaces. I think this is to reduce the air movement. In principal this depends mostly on how closely the springs are to ideal. One company, "Springfree" uses fiberglass rods isntead of springs. This allows them to put the rods under the frame, reducing the amount of frame you can land on, and eliminting putting your ankle between two springs from 10 feet up.
B: Sweet spot. Rectangular ones have a longer, but narrower sweet spot I think. This makes for a more predictable bounce landing on face or back.