The main diference (by my opinion) is that Fedora is somewhat older and so there is better guidelines for fedora. For ubuntu on the other hand there is much bigger community so there is much more chance that in nera future ubuntu will be (less or much) better. Look for example the brainstorm:
Ubuntu brainstorm

I think that this is excellent way to "talk" with community.

Also there are packages. Sanils already said that there is difference betwen packages and package managers. Apt is developed for a long time and yum still has some bugs.

Personally i perfer ubuntu but i belive that they need to do something with "weight": gnome should be a little lighter and ubuntu should be a little faster (for now i use my scripts to boost ubuntu but it would be nice to get this by default ;)).