Both nice, but I lean more towards the sound of Caleb with Theo (maybe it's the two syllables?).
But yes, both Elias and Caleb are more popular than they used to be if that a concern - but I really don't think they'll reach Theo levels as that boomed from Leo's popularity and the trendy "o" ending (Milo, Arlo...) . If you want uncommon, short Biblical maybe Asa or Jared?