If you think that Hp doesn't matter, then make it 5|0 or 5|10 (it is the same according to you).
A Cockatrice survives Paradox, Sockwave, Sniping, 5 turns of poisoning, 5 turns of Fire Shield, Rain of Fire, 2 Lighning Storms, Elite Otyugh etc.
A Giant Frog is Paradoxed, Devoured by Elite Oty, Sockwaved, Sniped, it does survive only 1 Lightning Storm and Rain of Fire instakill it, plus it survives only 3 turns of poisoning/FireShield.
I am a
hardcore player and I will say it again and again; 5|3 and 5|5 have a huge difference and well deserve the 1
quanta cost difference. I talk according to my experience.
You can't just use Omni Eaters instead of Cockatrices at mono-life of course, but as you said, you are going to use them at Aether/Time or Rainbows, or even at Mono-Aether MindGate decks which are fatter than normal. Actually, I can think of quite a few Aether duos that can take advantage of Omni Eater.