5107 has a noticeably flatter-profile bezel, and the (older) PP 315 Cal. movement. Gives the watch a somewhat more contemporary vibe.
5127 has a domed bezel (a bit more "classic Calatrava" many would say), and in current production models, the PP 324 caliber. I understand that the first 5127s were fitted with the 315, before Patek phased that movement out in favor of the closely related 324.
Best,
Adam