In light of the fact that the views from all of the tall buildings there are essentially the same, I suggest that you make your decision based on the food/beverage that best suits your mood. For me, the Suntory restaurant in the HSBC building (half as tall as the really tall buildings, but you still get a "view") usually wins out because it features some of the best Japanese food in China, but the bars in both of the Hyatts are also quite nice. Forget about observation decks; if it is smoggy, you might not get to see anything at all, but you can still enjoy your time if you have nice food/drinks to consume.