Companies like AOL and Compuserve use their own proprietary interface because they are not a true internet provider. They are a graphical bulletin board provider who provides a back door to the internet. The each do essentially the same thing, it comes down to what you started with. The one thing you MUST do continually is stay current. Microsoft's latest release 5.0 is much faster than their 4.0 release and can make browsing the web more enjoyable.
Many people download and install both.
Once for the kids and one for the parents. This makes it easy to limit content and sites the children can go to. The best place to download is directly from the manufacturer, however with 100 million users those sites can be slow.