City of the Spider Queen is the way to go.
I don't have Serpent Kingdoms, but I can tell you Underdark is both beautiful and resourceful. There are lots of new underground races, several campaign threads that I was previously unaware of (Deep Imaskari, phaerimms under the Anauroch Desert), and lots of stuff on the drow.
Plus, if you buy the campaign book, City of the Spider Queen, you have something to complement your Underdark book. Or rather, the Underdark book s a great compliment to City of the Spider Queen.
I am currently running the City of the Spider Queen campaign with one seasoned gamer, an experienced gamer, and a new gamer. They all love it. The story is great, the villains are tough and smart, and the lion's share of the work is done for you. Add into the mix that there is plenty of space to personalize it and I would recommend City of the Spider Queen over the Underdark book. Hands down the best DnD product I have purchased to date.
durin