The license is one benefit, yes... but the StrongLoop PM also has support for deployment across multiple machines and integration with nginx for load balancing. And everything can be monitored (including across machines) from a single GUI: StrongLoop Arc.

pm2 is great, but this is not an "added features" play, StrongLoop wrote this process manager from scratch based on experience with very large enterprise clients. In other words, this is a new beast entirely!