It's not down to price
It seems they both have advantages as well as disadvantages so let’s see.
Firstly, they both seal well, but aluminium fibre ones seal better if there are imperfections in the joint.
Aluminium fibre ones tend to be thicker and deform more to seal bigger deformations in the joint when tightened.
Copper ones are soft enough to deform and seal minor imperfections in the joint but are not as forgiving as fibre ones.
Aluminum fibre exhaust gaskets can be reused if they are not too old, but copper ones will almost certainly blow if reused.
Copper exhaust gaskets are easier to remove, you usually just have to tap them with a hammer or the tip of a screwdriver and they come off.
Aluminium fibre gaskets often corrode over years of use and will disintegrate once the exhaust is removed, leaving a mixture of white powder and pieces of aluminium that need to be chiseled off carefully.
There is no big difference in price as far as we noticed.
We recommend to replace the studs with stainless studs as well when they original ones are too old and knackered. Stainless studs can be removed much easier if the need occurs.