There are three drivers for ATI Radeon R500 and R600 based cards cards with 3D support on Ubuntu.
fglrx is the binary, officially supported driver which is suggested by Ubuntu to users on first boot (as a restricted driver).
ati/radeon is the open-source default Ubuntu driver. The Hardy version doesn't support 3D on R5xx/R6xx. It is independently maintained by the community.
radeonhd is the newest driver. It is developed by people who work outside but are paid by AMD, as well as developers from the the community. It shares the 3D component with ati/radeon but has different features in most other areas.
Now that all three drivers have 3D support (which is continually improving), which of ati/radeon and radeonhd should be the default for R5xx and R6xx cards in Ubuntu, and should fglrx continue to be suggested by the Restricted Drivers Manager if both of the open-source drivers support 3D and Compiz stably by the time Intrepid freezes?