Tom,
I guess I'm a 10%er; multiple times at NB and never at Muirfield, and my handicap used to be around 6. Deciding factor at first was ease of access and price and the fact that I was doing last minute planning. Subsequently, it was that NB is just so much fun, and I'm still doing last minute planning.
Ben,
Not sure where you are coming from or when or whether you'll be playing on your own or with others, but based on my experience (and I regularly do ridiculous 8 day circuitous tours of Scottish golf courses) I'd say that trying to do TOC and Royal Dornoch with 3 nights will be a logistical challenge.
If you are playing as a single forget about the lottery for TOC. Just go to the starter's hut and get on the list for an opening. It is even doable for a two ball in my experience. Don't forget to check and make sure the course is open for play the days you plan to be there. They do have a lot of fixtures.
If you are going in summer, you may well be able to play both TOC and NB on the same day.
If you want to go to Muirfield, you need to plan and book well in advance. Not sure when you were planning on going, but you may be too late to book already.
Both North Berwick and Dornoch are special golf courses. I'd highly recommend both. Hopefully you will get back to Scotland a second time to play whichever you missed and to play the myriad of other fine courses, both great and small.