I definitely would recommend working in 720*576. All digital formats (DV, SDI, DVD) use this resolution, so it has been the “standard” for quite some time. The only problem is that you cannot mix 720 and 768 media in the same timeline in Media 100 i (you can do that in HD and sw, btw.). So if you have projects that often need old material from earlier projects that would be the only reason for me not to use 720. But we decided to do all our projects in 702 and 48kHz some years ago, and this was a good decision since you can always use all media files in all projects.
If you want to export something from SW to Media 100 i 768 you should be able to do that using the Export->Other->QuickTime with Media 100 PAL setting. But quality will suffer doing that.