The bigger the better. From what I understand, discus are messy, so they need a lot of filtration. Also, they need bigger tanks, more than an xp1 can cover alone. I have an xp2 on a 46g, and I wish I had bought a xp3. I want to put more media in there than there is room for.
The xp1 has 1 basket correct? So you'd probably have room for the coarse sponges, a fine floss pad, then some biological media? (those stars or ceramic noodles. I personally use Seachem's Matrix).
I guess the more pertinent question is, how big is the tank?
Edit: Basically, avoid activated carbon in your filters unless there is a specific need for it. The carbon will absorb the nutrients your plants want. If you want a water polisher, Purigen does not absorb those nutrients that carbon will.