Well, Jinmao was impressive in it's day. It's still worth it if you can sneak into the hotel portion, I was able to in 2008 and looking down some 50 flights of stairs is pretty cool.

But, since the WFC looks down on the Jinmao, go to the WFC. Another tip I'd give is go on a weeknight and go late in the evening around 9pm. It'll be practically empty and you can take your time through the little exhibits before going up to the deck. It was quite a nice experience, one of the few times in China I wasn't pushed and crowded.

Dinner before hand in the lobby area is also a good idea.