Of the 120 strings we catalogue, 87 are polyester and 19 are multifilament — a split that tells you what the tour uses, not what most club players should. The two materials do opposite things.
Polyester is a single stiff filament. It lets the mains slide and snap back, which is where modern topspin comes from; it holds up to a heavy swing; and it is the least forgiving material on the arm. It also loses tension faster than anything else — a polyester bed is measurably looser after the first hour of play, and dead long before it breaks.
Multifilament is hundreds of fine fibres wrapped together, and behaves like a soft rope: it cushions the impact, holds tension better, and gives more free depth. It wears through faster under heavy spin, and it will not give you the bite a shaped polyester does.
The practical rule most stringers use: if you break strings more often than every ten hours, or you play with heavy topspin at pace, polyester is the honest answer. Everybody else — and particularly anybody whose arm has complained — is better served by a multifilament, and the money saved on the restring cycle usually covers the difference in price per set.
If you want polyester anyway, string it lower than you would a multifilament. Two kilos below the middle of the frame's published range is the usual starting point, and our tension calculator will propose a number for your specific frame and playing style.