It depends what you want to do.
Multisync includes TCP and basic firewall commands.
DarkNet has alot of functionality that Multisync doesn't. I'm currently working on a website which may persuade you to buy DarkNet
Here is a brief list of current DarkNet features:
- High performance TCP and UDP data transfer.
- Broadcasting; this allows an application to announce its presence on a local area network.
- 3 error modes (commands that return information about error, message boxes and exception).
- Multiple instances; this allows multiple connections and server hostings be setup at the same time within one application.
- Encryption/Decryption using Advanced Encryption Standard.
- Universal Plug and Play NAT traversal.
- Sound input/output.
- Windows firewall commands (much more control than Multisync gives).
- Compatibility with applications not made with DarkNet e.g. DarkNet can communicate with web servers.
- Asynchronous (non blocking) options.
- Easy to use.
- Secure.