which clock in Grand Central Station is worth millions, the outside one or the inside one...and WHY?

Answer 1

They are both very valuable. The outside clock over the entrance is made of the world's largest piece of Tiffany glass. But, the four faced clock that is over the information booth was estimated to be worth from $10 to $20 million by both Christie's and Sotheby's. The reason it's so valuable is that each of the four faces is made of precious opal.
http://travel.webshots.com/photo/2705433540046209414oAYrjD
http://www.workfriendly.net/browse/Office2003Blue/http/newyorkdailyphoto.blogspot.com//2007/03/passing-time.html

Answer 2

I'm assuming the one inside. Maybe because it's so famous (in many movies, etc.)