I personally despise the taste of fermented brown sugar with that awful molasses-y taste without sweetness. It's really gross to me, but I could probably stand it in a barleywine or some other very dark aged beer with plum/toffee notes. So right there, I would hate that second recipe because of the vile non-sweet molasses burnt sugar notes.
But your question was this: What would be the difference? As I understand it right now, the first one would get a large amount of alcohol from the base malt, leaving a relatively low amount of unfermentables, mimicking use of the sugar in the second recipe. Am I looking at it correctly?
The difference would be flavor, color, body, and balance. So, the whole recipe changes when the recipe changes. The body may change, or may not. The first beer would probably be a medium bodied beer, with the "American" flavor. The second beer would be "thicker" with the higher mash temp, with odd toffee flavor from the fermented brown sugar. They would be totally different.
