Skullcrushers are partial movements.This exercise is not for everyone, though a good amount of weight can be used after awhile. Just my view, there tends to be too much prolonged tension on the elbows/joints with what ever style of skullcrusher you are doing. If it would be important for you to do a full range of motion, than try triceps extensions to behind the head, as The Pumpster suggested. Full Tricep extensions are best done standing or sitting. BB, DB's,EZ bar, cables, manuel resistence, etc. In any version, some people will use a curl grip for a different feel to the exercise. Some even a tricep bar.
What version (incline, decline or flat) of SC's is better depends on you and the results you get, without discomfort. And as was suggested before, give each style a fair try. Good Luck.