When using this style of wedge for sitting you may want to consider a higher density and firmer foam, than a back or headrest.
Keep in mind that if you are sitting on it, it will be supporting your entire body weight, you may want to consider a 2 lb foam grade or higher. The heavier you are the higher the density should be.
If you are sleeping on it or using it as a backrest you can go lower in density.
A 1.5 or 1.8 lb foam grade is a good option.