Does Apple Remote Desktop send framebuffer data or rendering commands? Does Apple Remote Desktop send the framebuffer (VNC like) when using remote viewing/control capabilities or does it send rendering commands (RDP like).

Does this differ based on what client is connected (VNC vs. the ARD client)?
VNC like. It has several compression schemes, but things like the startfield animation drive it to high bandwidth and very low quality.

This is one area where Windows just kills Mac. I couldn't believe how much more responsive rendering commands were when using RDP. Even windows XP is so much faster for remote access. I can work all day remotely with decent fast ethernet and a fast TS server. Not so on Remote Desktop unless all the stars are "aligned" and off screen so to speak.