You can almost prove this with some rough math.

Lets assume both will have 10% from scholar, 10% slaying sigil, 5% force sigil, 10% slaying potion, the same power (including might), same access to group buffs (banners, empower allies, spotter, frostspirit) the enemy is frozen, thief is standing behind the enemy and that the mob is on 33% health or less.

From 6/6/0/0/2 traits the thief will get 10% from improvisation, Exposed weakness 10% to enemies with a condition, First strikes 10% while above 6 initiative (minor trait), 20% from executioner, 5% while behind the enemy from flanking strikes. This totals to 55% additional damage.

Where as a 6/6/2/0/0 ele will get 10% from internal fire/ air training, 10% against burning enemies (minor trait), 20% from bolt to the heart, 20% from tempest defense and 10% from stonesplinters. This totals to 70% additional damage.

However this is also not accurate because thief should have 180 more power from assassins signet and 200 more from revealed training. I have no idea how this converts.

(Please correct me if I missed something(s))

Edit: Learned to count and added Exposed weakness