What is to gain from going to Fedora from Ubuntu?
I'd say that depends on your perspective. You'll mostly be using the same stuff, but potentially:
1. Larger number of official packages -- you get a lot farther in Ubuntu without adding additional repositories, whereas with Fedora you tend to need Livna, RPMforge, rpmfusion, ATrpms, etc. to have a complete system due to their more aggressive ban of "non-free" stuff and smaller set of packages they've agreed to officially maintain.
2. Larger community -- this can help with getting support or answers to common problems.
3. Less breakage -- Fedora pushes the envelope when it comes to new features and moving to new versions of stuff too aggressively IMO. They do it to the point where something major is broken in ever release (binary kernel modules, pulse audio, selinux policy, etc.). Ubuntu is less aggressive, but lately I've criticized them for doing some of the same things compared to vanilla Debian.
4. Better package system -- this is subjective, and yum has largely eliminated many of the advantages to apt, but from my perspective it is still a lot faster to apt-get install something than yum install it, and definitely faster to apt-cache search something than yum search. apt+dpkg is definitely less fragile than yum+rpm. This is slowly improving, though. One the other hand, Fedora gives support for split 32/64-bit systems with less hassle, although the need for that is less important lately.
Ultimately, though, if you're more familiar with how to administer Redhat-style systems, switching to Ubuntu will take some re-familiarizing in where configuration settings are and how stuff works.