Voodoo Extreme published some details about the differences between the DirectX 9 and DirectX 10 version of Crysis in multiplayer. What it all comes down to is that you'll be wanting to play the DX10 version as it offers significant advantages:
To shed some light into one of the most discussed topics regarding Crysis multiplayer I would like to explain you the differences between Crysis MP DX9 and DX10.
As for the DX9 version we won’t have physics and day and night cycle in-game. That means you won’t be able to shoot down trees and/or alter any other objects than vehicles on the map. Additionally the time of day setting doesn’t change dynamically. This is caused due to the tremendous server load such physics might cause on crowded gaming servers. Still you will be able to experience maps with different time of day settings since the maps can be altered in the Sandbox2 Editor.
Rather than providing the community partially working features we limit this for the DX10 version only. Due to the strong hardware available with DX10, server load is less and performance is increased. This ensures the pure physics and day and night cycle experience without any limitation.
Gamers with a DX10 card are able to play on DX9 servers, but with the limitation of the respective server. Vice versa it is not possible for gamers with DX9 cards to play on DX10 servers due to the limited features.