you cant beat playing an ele sham in eots or being at LM in AB. but i hate playing my ele sham in arenas. they always end up sitting on me and im the first to die. i like both for playing in bgs but i like playing shadow ALOT more in arenas.
either has absolutely insane amounts of damage right now. shamans mastery is by far better, but priests have some useful utility, such as defensive dispels (only resto shamans have def magic dispel). i personally would play a shaman simply because of how insane their mastery becomes with a lot of +mastery gear.