I was a beta tester for Hex and recently got Silo for work.
Here's the main difference for me between the two:
Silo seems to be geared totally for box modelling whereas Hex is geared for box modelling and modelling from polylines. If you are someone who imports Illustrator paths to build your models then you are out of luck with Silo. The specs on the Silo upgrade doesn't mention Illustrator import either. Hex on the other hand will import Illustrator paths(although not ones originally made in FreeHand unfortunately). Hex also has deformers which Silo doesn't.
Hex has a fair bit more functions than Silo but you pay a price for it: It costs a little more and requires faster hardware than Silo. Don't get me wrong, Hex ran fine on my G4 mac but Silo whizzed through anything I threw at it with ease. I think both are fine modellers as is modo but when you pay as much as they are asking for modo it should be better.

Message edited on: 08/30/2005 17:38