Tough call.
GT your sim can go to dance parties and clubs, the club system is great and can be used to your advantage. It seems to get updated with every pack that comes. The clothing in GT is great from casual to party and the world that comes with it is big if you're also wanting more space for your sims. Sims having more places to go and things to do is always a plus. Especially when they can go to cafes as well.
Seasons is just a great addition since sims and surroundings are affected by it and sims being able to celebrate actual holidays with traditions and you can make your own. I really love the furniture and decor that comes with it. Just having weather in the game really changes the atmosphere. Weather can make the day better or just ruin it if your sims have something planned like a pool party or something else.