- better .tar.gz handling: Winrar treats it as one file, 7zip as a tar in a gz that have to be extracted separately
- recovery records to be able to recover from mild bit rot (basically built-in .par2 without the mess)
- support for more NTFS attributes and features (saving alternate streams, security records, storing hard links as links instead of separate files, etc)
- can do useful things with the archive attribute of files if you so desire (archiving only files with the attribute set, clearing the attribute after, optionally deleting them)
- better GUI (the style is a bit dated, but the 7zip UI is just an incomplete WinRAR clone with smaller icons)
Overall WinRAR is the much more solid archival tool. If all you need is something to decompress archives WinRAR isn't too different from 7zip, but has a slightly nicer UX.