When going to DVD Studio Pro then Exporting with Compressor works better for me. It retains the 16:9 just fine for 1080 and 720. I have not tried to squeeze HD into 4:3, but there are settings in Compressor to do that. I have also mixed 720p and 480i "compressor" produced files in DVD Studio Pro. You can tell one from the other by the contrasty look of the 480 next to the what used to be the 720 file. But the aspect ratio is 16:9 as set in Compressor.
Look again at Exporting to Quicktime. There are a few target formats to try.