As a Tempest of Set, it's actually rather difficult and somewhat confusing to pick which feat tree to specialize in. Thunder focuses more on support abilities and buffing, but there are a lot of interesting utility damage and crowd control spells in the mix as well. Lightning has more powerful area of effect abilities, but it's interspersed with regeneration bonuses, party buffs and more. Higher tier lightning abilities like Puppet cause area of effect confuse status ailments on enemy targets. Serpent Transmutation, the top spell in the thunder tree, makes it so that most damage incoming to the Tempest of Set during the spell's effect heals them instead of doing damage - very powerful in PvP, as the Tempest can then continue to heal their party even if they are focus-fired by an opposing raiding party.
A Tempest of Set should not neglect their priest abilities, either - some abilities, like Exemplar, which boosts all melee, lightning and holy damage, scale well with gear.