Difficult to say what will be better in that scenario, but if pushing ISO to the max is a common use case for you, maybe full frame might be worth considering, as obviously the light gathering there would help you no end.
It does depend on whether you are after record shots or National Geographic shots too, I always go for the former, hoping for the odd latter!
It will be interesting to see some head to heads of the final production model.
I have never considered the OM-1 as I really like Panasonic bodies, I realise each have their strong points, but knowing you have the best of one manufacturer does make it easier to choose! I don’t think either will be a bad choice.