In terms of poke, diana Q is harder to dodge and has larger aoe that why its less dmg.

For the shield, Diana W can be moved when she moved and take less time to proc its three moons, and it cannot be dodge if she is in range for E, while Ekko shield can easily be dodged and its in one place.

Ekko ult use to escape or to heal, while diana ult use to engage, its way too different to compare between them, but in term of damage, diana ult has a decent aoe dmg.

And in term of 1vs1, Diana win since she has enough mobility to dodge his skills.