Which to buy?Underdark or Serpent Kingdom?

[sellars]

If you really have to choose, I'd say go for usability, since both books are equally good (IMHO).

Choose the one you can actually use the most in your campaign.

 

[BigFreekinGoblinoid]

Both are good books, but you would have to spend a LOT of time underground to get full use from a book that details as large a space as The Underdark. Underdark overall has more stuff ( more races, more cities, more geography ). Serpent Kingdoms has some of that too of course, but I'd say more detail on what it does cover. Serpent Kingdoms could definitely be a great resource for detailing an ongoing serpentine campaign menace in Eberron.

You really can't go wrong here though. It's really more of a stylistic question for you to answer yourself.

edit: The Underdark was released right before the 3.5 update i think...

 

[Shemeska]

Both of them are very good, but SK knocked my socks off. I wasn't expecting it to be as good as it was, and I'm happy I bought it. Despite not running an FR game, per say (running a 3e Planescape game), I can get some use out of it besides just reading it.

 

[Mystery Man]

Woo-hoo, got both

Someone else with no self control. Glad I'm not alone.

 

[Pants]

edit: The Underdark was released right before the 3.5 update i think...

Right after.
I gave the book a pretty good read through (my friend owns it, not me) and I don't remember seeing any 'glaring' revision errors.