Ubuntu is based on Debian GNU/Linux, a free (as in free beer and free speech) Linux-based distribution and the free GNOME desktop environment. Therefore it keeps the phylosophy of the two, being itself also free. Summarizing, this means that the user can legally modify and copy the system at will, without having to pay anyone for doing so. When things break, it is great to be able to look at the source code, find the problem and fix it yourself; of course, this is not something that end users will ever do, but I have found this situation valuable many times (not under Ubuntu though).
Mac OS X, on the other hand, is a proprietary OS with the exception of the core kernel whose source code is published as free sofware (I don't know the license details though). This means that you must pay for a license in order to use it, and even then you cannot mess with its internals — its source code — nor redistribute it. Given that Mac OS X comes prebundled with new Apple machines, this is not so important because you'll rarely feel the need to look at its code (I certainly don't care as long as it works). However, if you want to jump to a new major version, you must pay for it. For example, if I got an iMac now, I'd have to pay around 200€ in mid-2007 to get the Mac OS X 10.5 family pack (5 licenses); I'm not implying that it's not worth it though.
I know the free software ideals very well and like them but, sincerely, freedom is something that end users do not perceive in general. And I won't base the decision on which OS to run on my computer based on this criterion alone; that's why the iBook is stuck with Mac OS X ;-) Really, I've lately come to think that what really matters are free and open standards (i.e. communication protocols, document formats, etc.), not the software packages themselves.