K2 adds a bunch of advanced factory stuff. If
Krastorio 2 doesn't appeal to you it's not needed.
Depending on how you plan on playing, because of the upcoming space changes I don't even know if I can recommend Space Exploration until it adopts the changes. I think a typical playtime to beat SE takes some 600+ hours. Personally I'll probably end at ~800 hours spread over three years :| If your experienec is like mine, SE might be vastly improved by the time you start and you'll have to choose between "old" and "new' SE since a restart will almost certainly be required.
If you're the kinda guy that will bang out 600hours of playtime in three months, now isn't a terrible time to do it.