Hi audball, I use sour milk (milk and vinegar), plain yogurt, and sour cream interchangeably
in all my baking, using yogurt the most often. Everything comes out well.
As far as the Chocolate Sour Cream Bundt Cake is concerned, I've used both sour cream and nonfat plain yogurt with no noticeable difference between the two, but haven't tried it with the sour milk. Some people find that yogurt and sour cream in general yield a richer result in baked goods, and others find that buttermilk or sour milk yield a more tender result. However, I doubt if most of them have done side by side comparisons, so they really can't tell how much of a difference, if any, there really is. I find they all work well.
If you have plain yogurt, I would use that as a sub for the sour cream, if not, I would go with the sour milk. Since they all can be used interchangeably, there shouldn't be any problem.
By the way, your username sounds familiar. Were you ever on a forum called "Discuss Cooking?"