As you probably guessed some time ago, neither Fedora nor Ubuntu has any major advantage over the other. Behind the scenes, Ubuntu may use Upstart instead of Init, and Fedora may speed its boot time with Plymouth. But to the average user, these changes are largely invisible, and tend to cancel each other out if you notice them at all.
The same is true of the latest Ubuntu's use of GRUB 2 of the newest Fedora's use of ABRT for reporting bugs. Generally, only advanced users will notice the differences.
The only way you can make a meaningful choice is if some feature is of special interest to you. You might install with Ubuntu's alternate installer if you are having trouble getting any distribution onto a particular set of hardware, or go with Ubuntu because you need easily configurable non-English language support.
Similarly, you might choose Fedora because of the peace of mind that SE Linux brings, or because you want to use fingerprint authentication. Someone who feels strongly about the use of Mono would probably want to avoid both in favor of something on the Free Software Foundation's list of free distributions.
The truth is, given the mature state of the free desktop and each distro's undoubted wish to match the features of rivals, it is becoming increasingly harder to find features that make one stand out from the other. There are still significant differences in desktops. But when distributions use the same desktop, the way that Fedora and Ubuntu do, then the differences are likely to be unnoticeable to three out of four users. These days, you are even unlikely to find any differences in speed or stability unless you have some unusual hardware configuration.
That may be an unsatisfying answer to those who like to pick a side and defend it. But look at it this way: the lack of a clear victor shows the general sophistication of free software today. Now, in most cases, you don't have to choose between major distributions -- no matter what your choice, it is likely to be a reasonable.
ALSO SEE: Open Source Downloads: the Monster List