I can’t speak to how they really compare, or the accuracy, but if your oldest child is in 2nd grade, I wonder if both of those choices are going to be too much. I just looked over The Story of the Middle Ages as a possible spine for our next run through medieval times, when my youngest will be 2nd grade, and I decided it would be too advanced for her, but just about right for my will-be-then-8th grader.
I do not have A Castle With Many Rooms, but the chapters look long – too long for a 7yo, in my opinion. There is a whole chapter in the sample if you want to read it over and see what you think. For me, I would be more inclined to do something like Story of the World 2, or even just medieval-themed picture books, like the ones SCM has scheduled in the Middle Ages guide.