The main difference is in the lewd scenes - different actual scenes and/or dialogue depending on route. Some differences in minor events between routes as well. There is also a fundamental difference in MC's relationship with Cathrine on both routes.
I feel the corruption route is decent, but in the end standard stuff seen elsewhere. The seduction one is endearing emotionally and fits the general narrative better as the developer spends significant time to build up MC's perspective in retrospect as a middle aged guy time travelling to the past to fix past wrongs.
Seduction is not 100% romance either as it has elements of corruption as well, just not that pronounced.