Having read your comments, I came to this conclusion:
I'll have a try with 7zip and if it compresses slightly less or better
than rar, I'll use 7zip (because it is free). For a test period I'll
provide both, zip and 7zip/rar and the user may decide which one will
win. Additionally I'll provide a file 'how to decompress.txt' in every
release. Wheras I think, people who come to download an OS know enough
to find one theirselves. Making exe-files is still an option but I
don't like it. So you have to convince me.