There are 2 main installation options: the online and offline installers.  Both will provide  identical versions of Alice.  The difference lies solely in the method of downloading the bits.

Online Installer

The online installer by itself is a relatively small file (~3MB).  After downloading and launching it, it will require an internet connection in order to pull the latest version and install it on your computer.  While the entire Alice installation requires 500-600MB, the benefits of the online installer are:

1. Quick initial download
2. Faster updates, as it only pulls what is needed

The downside includes:

1. The machine has to be connected to the internet, without interruption, for the entire duration of the installation.  
2. The online installer does not provide a copy of the full download for a local backup.

Offline Installer

The offline installer contains everything required to install Alice.  It is a large file (~500-600MB, depending on operating system).  The benefits include:

1. After initial download, no internet connection is required to install
2. You have a copy of the full version for backup, in the event you want a full copy of the installer on your local computer.

The downside includes:

1. The initial download will take some time, depending on the speed of the internet connection.  
2. For upgrading a previous version of Alice, in most cases the upgrade could have been done faster via the online installer (which pulls only what is needed for updating, rather than everything).

Note: It is possible to update any past version of Alice 3 with either the online or offline installer.  If you initially install Alice with the offline installer, you can update with the most recent online installer, and the other way around.

Relevant links:

Updating Alice

NetBeans plugin

Installing Alice

Installing the Java JDK