Shattered Kingdoms

I've been told that casting petrification you gain experience from the spell itself but not from the kill, if this is true than does the same apply to Finger of Death? If not why doesn't it?

Or can someone explain to me experience ways how both these spells works.

To put it simply, you get combat experience by doing damage. Finger of death hits you with a ton of damage, so you get XP for that. Petrification doesn't do any damage, it just kills you.