Sunshine got a more complex relationship system. If you want to enjoy (or hate ?) the "building relationship", it's better and take longer to get intimate (in koikatsu you can end lover in just 5 days, game time, by getting all the right action / answer).
Koikatsu got more more and more mods to customize your girls.
Sunshine got the island and romance mood instead of the school harem of Koikatsu.
Make your choice.