ESS is still in development. One thing to consider is if you would like to have Multiplayer.As yet it is uncertain if or when it will be in ESS..
If you are after a more realistic sim sse is the one to go for. There are a lot of people who use SSE who are also involved with the sea and the community and multiplayer are (usually) realistic.ESS seems to be communal and doesn't look as realistic!
Well, SSE has more ships, sceneries, missions, etc. ESS is newer but according to some videos I've watched, it doesn't have many ships, missions and sceneries. However its graphics are satisfactory. YOU are the only one who can decide which game you will play... Watch some videos on youtube with both games and then decide.
You bought both? Hmm I guess that's another way of solving it if you don't know how to decide