I dont know if my post answers your question, therefore I'm going to express myself in words.
Sun Serpent can do a little more damage than efreet, and gives a little pain to the other enemies. As a pyromancer, this spell really looks great for me because you can use the 1000 damsge on a boss and the other AoE to the minions. No need for an Dragon and then an Efreet anymore!
Skeletal Dragon has more damage than Avenging Fossil, but if we add the AoE damage to the single target damage, we can have 1 200 Death Damage, which a small damage increase from the Skeletal Dragon at 1 010 Damage if we add up all the DoTs.
So, to answer your question, the conclusion is that yes, they both do the same effect, just different amounts of Damage.
