Ian,
Thanks. I have the same choice as you - a light pertex jacket and a bulky OMM jacket. Problem with the latter being that I will struggle to fit it in the bumbag with a bladder so may need to run with a sac instead (of course if it's going to be cold and wet I won't need as much water:)).
As you say, best to be prepared so I just pack everything and see on the day.