Then the problem becomes...

Weighing 1 death compared with 10 deaths seems obvious: The best thing to do is to allow the 1 in order to prevent the 10.

That is weighing death with death.

But how about suffering with suffering?

There might be two relevant dimensions on suffering:
1) Intensity
2) Duration / Number of People

Lets say that with respect to intensity we can rank intensity of suffering on a scale from 0-100 (where 0 is no suffering and 100 is the worst suffering imaginable).

Duration / Number of People is a bit tricky (I think it will work out okay to put this on one scale). The thought here is that 10 people suffering of intensity 50 for 50 years would be equivalent to 20 people suffering of intensity 50 for 25 years. Or that 10 people suffering of intensity 50 for 50 years would be equivalent to 5 people suffering of intensity 100 for 25 years. (I hope this seems plausible).

(There are issues with trying to quantify things in this - and in related - ways...)

So... The omelas are suffering...
But in our present society (without omelas) people who aren't omelas are suffering.

If we add the suffering of omelas then is it better or worse or much the same than the suffering of the non-omelas in current societies?

And with respect to fairness... Is the choice of who will be an omela any fairer or any unfairer than the choice of suffering non-omelas in current societies?

The plot (moral dilemma) thickens...