Historically, Ubuntu focuses on the user experience such as beautiful graphical user interface, while Fedora focuses on performance. In the other words, the kernel in Fedora distribution is more up-to-dated.
I just upgraded by Ubuntu 10.04 to 10.10 (released on October 10, 2010), I found that the Linux Kernel in Ubuntu is more advanced than Fedora. I am surprised!
#cat /etc/lsb-release && uname -a DISTRIB_ID=Ubuntu DISTRIB_RELEASE=10.10 DISTRIB_CODENAME=maverick DISTRIB_DESCRIPTION="Ubuntu 10.10" Linux 2.6.35-22-generic #33-Ubuntu SMP Sun Sep 19 20:34:50 UTC 2010 i686 GNU/Linux
and for Fedora 13:
#cat /etc/redhat-release && uname -a Fedora release 13 (Goddard) Fedora release 13 (Goddard) Linux 18.104.22.168-56.fc13.x86_64 #1 SMP Wed Sep 15 03:36:55 UTC 2010 x86_64 x86_64 x86_64 GNU/Linux