Your choice should be based on whether you need a physical or special attacker; neither set is a must-have over the other.
i personally believe that the physical set is Serperior right now; the turn of the generation has kind of made Contrary Serperior...not obsolete, but not as great as advertised.
With Coil, you do have to set up a bit but you can give and take physical-heavy hits, which is perfect to help you deal with the threats of the physical-heavy meta (I'm looking at you, Talonflame). Also Grass+Water+Dragon is sexy coverage, and if you have hazards then you can spam Dragon Tail for days and win that way.
The Special set, while it's undeniably powerful, is weak defensively, and Serperior being a Grass-type only amplifies this problem. Leaf Storm is actually easily switched in on by pokes like Aegislash and fliers. Also 325 speed is, quite simply put, not enough; Salamence, Charizard, and various megas have you served on a silver platter.
i personally dislike Serpeior in sun over Victreebel / Venusaur, but don't be afraid to do your own thing!