5-10 years ago, there would have been some defined and clear cut answers to that question. Siemens programmers did things one way, and Rockwell programmers did thing another.
Nowadays, both companies have made improvements/modifications over time that have incorporated each other's ideas, and have addressed at least some of their individual weak points.
The biggest differences I see now are in the supported communications protocols, and the terminology. Rockwell's main Ethernet communications bus is Ethernet/IP, whereas Siemens uses Profinet. They do essentially the same job, just different approaches.