In a rectangle trampoline, the criss-cross setting of th esprings gives a uniform bouncing surface over the trampoline. The springs work at a differnet rate to give a quicker response and a more poweful lift. A rectangle trampoline are great for Gymnastic use as they give a more powefull and specialized bounce which gives the gymnastist more time to perform trampoline tricks in the air.
A rectangular trampoline is therefore more suitable for professional sports training for professional sport.
Being rectangle means the trampoline has more surface area then round trampolines for their size.