Unfortuneately the better a fabric performs in one are usually causes perfomance in another to decline. Many cheaper shells are made with a polyester outer fabric.
Polyester has two favorable properties.
Unfortunately it is not very durable. Nylon has an excellent strength to weight ratio. Nylon fabrics are made of filaments twisted into yarn of varying weights. Some companies measure this in the form of Denier. Some packs come in Denier up to 1000 in areas that suffer a lot of abrasion.