I recently made cupcakes with Ghiradelli bittersweet chocolate and baked them for about 18 min. and they turned out moist and wonderful. A few days later I did the same recipe with Valrhona bittersweet and baked them for the same time and they were a bit drier. Was it me or do different chocolates require different baking times.

Different brands had different amounts of cocoa and cocoa butter. So it is entirely possible. Although I interchange brands all the time and have not experienced this....yet.