When you say gameplay, do you mean like just the story stuff or does sunshine have less scenes and positions available too?
With mods, it has the same or more as the modded KK.
The issue with KKS is, that it is just a sidegrade to KK, and the difference boils down to different location and that you have to grind the ever same "topics" for conversation on the ever same map spots.
There is no "koikatsu club" or so, where you could invite the girls, like in KK.
But you can give them a massage now.
I also think, that the dating-mechanic was removed for every personality except those being added with the DLC.
Since the map is larger in space, you will also have to search for the girls more often because they are not following a shedule. (But there is a teleport function for that in some mod, I think.)
Thinking about it, I really wonder what "Koikatsu 3" (to be announced within the next 24 hours, roughly) will be, and if we finally will get an actual game to play and not just a "click until bar is full, then shag" app.