Philip,
I would love to hear what you think of those two courses. I was greatly disappointed at not being able to see Prestwick on my first trip over.
Neil Regan is a member at Machrihanish, so I would contact him to see if he might be around at the same time you are considering being there. He may be ready for a vacation after all the work he must be putting into getting ready for the US Open at Winged Foot. I have always wanted to see Machrihanish and after hearing Neil's enthusiasm for the course when we talked about it, it will be a priority on my next trip.