Well, I'd say first and foremost, it's a matter of preference. For example, some people just can't stand yum, others love it.

From a technical perspective, Fedora tends to be about 1/2 to 1 release ahead of Ubuntu, by which I mean the software in Fedora now will be in Ubuntu in 3-6 months, and has less "magic".

If you care about free/libre/whatever it's called today software, then Fedora tends to be a better choice than Ubuntu, since the Fedora developers tend to work much more closely with the community, whereas Ubuntu does its own thing.

These days, Fedora uses Gnome 3 whereas Ubuntu uses Unity for its desktop environment.