There are many compression formats, and the one you should choose depends on what files you’re zipping up. Typically you’ll want to use a zip format for a group of files that need to be compressed so they’ll take up less space because it can typically compress at least twice as well as most other formats.
Another consideration is whether or not the file needs to unpack/unzip without launching an application. Here WinRAR has an advantage over 7-Zip since it can create executables and self-extracting archives, but 7-Zip still has some advantages with its increased compatibility, additional archive handling options like encryption and solid archiving, GUI interface (if you need to do everything at GUI), clean and lightweight interface (less eye-candy, but easier to navigate if you’re older like me!)
A final consideration is whether or not the file needs to be compressed efficiently. Here 7-Zip has an advantage over WinRAR since it uses LZMA compression rather than RAR5 compression which is not as efficient, but keep in mind that if you’re going to be compressing a large group of files together then the size of the output file from 7-Zip will likely be larger.
