Why bother with RAR at all? Zip is far more popular, but if you want the better compression ratio, 7Zip would be the better option. Also, what you have to keep in mind is the target audience. I think people who are interesting in downloading source code for ReactOS are going to have 7Zip handy tools, or know where to get them.
Of course the last thing you want to do is scare someone off because they see the .7z file extension and run for the hills. Why not trial both archive formats?