What is the difference between the several GNU/Linux distros like Ubuntu and Fedora? Before brickbats fly for asking an oft repeated question, I am talking of differences related to the internal working of the operating system, not eye candy, desktop environments, package managers, ease of use and other user interface related features.
To be more specific, suppose I am purely interested in the performance of a certain C++ program (serial or parallel) that I have written. Say I have Ubuntu and Fedora installed on the same desktop machine, with both having the same Linux kernel version, will the program performance be the same on both the operating systems?
A related question would be why some Unix like OS'es like FreeBSD (which is not a GNU/Linux distro) are more favored for server platforms than others.