It depends. Always author at the highest possible resolution you think you will need, because its far easier to scale/crop down than to redo the project. For most HDTV, this is 1280x720.
I think your best bet is to author at 1280x720, BUT... since you know this is going to end up on TV, you probably want to use a safe frame with 4:3 aspect ratio rather than 16:9. In other words, don't put any text or important elements outside the 4:3 safe frame, cause you may end up cropping the 1280x720 down to the PAL dimensions.