The W is far better than anything in Nusa Dua. Some fab bars/restaurants near it, too. Consider Sardine, Ku De Ta, Metis, Sarong, La Lucciola. All fabulous, in different ways (some more for food, others for partying until dawn).

Kuta is in general backpacker partying, cheap and rowdy, while Seminyak is much more upscale (obviously lots of variation in both places, but gives you some perspective).