The difference between Line Bonding and Load Balancing is the maximum speed and address range.
With line bonding you can download a file (using a single download thread) at the maximum line speed, eg. 8Mbps if you have 2x4Mbps lines. Also, the server you download from will only register one IP address as yours.
With load balancing a single download can only run at the maximum speed of a single line, eg 4Mbps. Sure you can download at 8Mbps if you use a multi threaded download manager, but any single download will only use one line. The other problem is that each line you use has its own IP address, so one moment you communicate to a server using one IP address, the next moment you use another (as far as the remote server is concerned). This can be a problem with some sites.
The nice thing about load balancing is that you can usually use any ADSL ISP, so you won't get shafted with "reasonable" prices like they do with line bonding.