Duct sizing impact on flow
In either case you have a 10" duct as the bottleneck in the trunk line. Expanding it downstream won't get you much in terms of flow. I'd go with Plan A.

Perhaps a more important question: Is the inlet attached to the fireplace somehow? You'll probably be disappointed if you just pull air from one room to another. The differential would be so small that you won't see much benefit. In fact, the air movement may make those rooms actually feel cooler.