Please help me understand Networks and WP Multisite.

I am looking into different types of network setups. I can see 3 basic ways to setup multisite:

1. Regular WPMU (sub(domain/directory))
--->This allows for multiple sites for you and your users.
2. Multi-Network:
--->This allows for multiple Multisite Networks of (Regular WPMU installs (#1))
3. Networks +:
--->I believe this allows for Nested Networks like the Multi-Network plugin, however it adds a custom table called "domain_mapping" in the database.

Question 1: Can anyone please give me insight into these different Multisite methods?
Question 2: Specifically, is the handling of the database different for each method mentioned above?
Question 3: Is method 2 or 3 generally more compatible?

Thanks again