Cartel and CustomX are leaning to the stiffer side...but not a 10.
Your weight plays into it as well. I do find that what a 130 lb rider calls "medium" feels like a pair of slippers to my 200 lbs. A burton driver X or K2 Maysis (both "stiff") feel medium to me.
Point is, it's subjective. Get what feels right to you. Flex them by doing some deep knee bends, pay attention to when your heels come off the ground and how much weight it takes to make that happen. Something like a burton hail, which is rated at 6/10 stiff, feels like mush to me. My son, who is 40 lbs lighter, finds them just right.
As to what fits the bindings, what size boots do you wear? If you're an 8-11, and your cartels are medium, they will fit. Adjust the bindings right and your good to go.