Balance druid is very nice, wrath spam is very cool, starfire hits like truck and You have great aoe ability with very low cooldown - starfall.
Shadow priest - just dot everyone. They have disperrsion so its nice defense ability aganist meele that hits like truck.
Afii lock - quite similiar, just dot everyone. Pet helps You with absorbs/silences etc, but managing him can be burdensome. Honestly I prefer destro lock - nice big crits every few seconds from chaos bolt and very cool aoe stun shadowfury.
Best way to decide - just log on the Blackrok realm. You can make instant 80 with full relentless gear there. Make druid, lock and shp, play few bgs and it will help You decide for sure.