[table][tr][td]RAR has a problem. This format is a proprietary format, meaning other companies other than RARLAB cannot create RAR format file for free theoretically, and its RAR module cannot be freely distributed across multiple other platforms, especially *nix and Linux platform.
7z, however, has very wide support among various Linux distributions. Mac OS X has 7z support out of box. 7z itself is also a free software, free to distribute.
I don't think there are much differences between them basically, other than platform support and license types.
Note: RARLAB is the genuine original distributor of WinRAR. Please download WinRAR from this site: http://www.rarlab.com