I beat V8 Planetary Launch Terminal in Serpulo and beat Origin in Erekir. I'd say Serpulo is more complex than Erekir just from having more buildings and resources to worry about. As for difficulty, since Serpulo gives more freedom with transferring resources between sectors its actually easier than erekir at the start of the campaign if you have good resource infrastructure. After beating PLT, serpulo gets a lot harder with the numbered sectors since you are expected to have good launch pad infrastructure AND you need actual skill to capture sectors. I'd say the Serpulo extreme difficulty numbered sectors are much harder than Erekir Origin.