So in terms of what do you select need to setup your linux ? for learning purpose only ? or for production environment ?
In case you do need a linux for learning purpose, I will select red-hat at first and then try out other distribution like gentoo or debain.
In case you do need a linux for production environment, you have to specify your requirement like what kind of server you are setting up, what kind of package would you need to install, etc. e.g. Weblogic or Websphere will have certification program which they will only cert on certain branded distrbution like SUSE or Redhat. By the way, you also have to consider the cost of purchasing a support license in this case. Coz most of the branded distrbution require you to purchase yearly support license which is quite expensive in some sense.
This two are good site which you can obtain a lot of resources for different linux distribution
http://www.linuxiso.org/ http://distrowatch.com/