The "Slim Build" leaves out the optional third party offers in the installation process that fund the distribution of the free version. It was originally created for folks who were getting false positive alerts on the Google Toolbar offer. Although the Google Toolbar offer was removed as of version 5.58 (back in June) we have retained the Slim Build for vision-impaired users whose screen-readers may have trouble with the extra information on the installer screen, and handle some additional installation problems.
Should generally try the full version first though. The Slim Build is often released at the same time as the regular build - but not always.