I suppose when you're talking about party, you mean the standard koikatsu because the actual party version isn't modded and sucks compared to the standard one. That said, they're basically the same. Personally I like more the sunshine version because it's a bit more polished, but that's it. The standard version has a basic dark mode dlc with a little bit of MMF content but it's very standard, you can't even change the second man who is a standard janitor. Now, if you don't care about the Studio and the free mode, you might care about the story and the gameplay.... I never played them to be honest, but the first one is set inside a school while the second one is set inside a resort or something like this.