As much as we may like free software, there is a lot of interesting commercial applications out there (be them free as in free beer or not). Given the origins and spirit of each OS, the amount of commercial applications available for them is vastly different.
Let's start with Ubuntu (strictly speaking, Linux). Although trends are slowly changing, the number of commercial programs that are addressed to Linux systems is really small. I've reasons to believe that this is because Linux, as a platform to provide such applications, is awful. We already saw an example of this in the software installation comparison because third-party applications have a hard path to distribute their software under the Linux world. We will see more examples about this soon in another post.
In my opinion, this is a disadvantage because, although there are free replacements for almost any utility you can imagine, they are not necessarily better yet. Similarly, there are tools for which no replacement exists yet. Or simply put the user may want to use such commercial tool because he prefers them over any of the other alternatives.
On the other side of things, a typical user will generally be satisfied with all the free tools included in the Ubuntu repositories. If not, sites such as Sourceforge or Freshmeat are full of Unix-based free applications. Generally they won't ever have the need to consider commercial applications so they won't have to spend any single amount of money to use their software nor keep it up to date.
Mac OS X is a different world; commercial software (shareware, freeware, etc.) is still extremely abundant in it. This is probably, in part, because the platform is also commercial: developers won't feel "strange" in providing applications following the same model, and there are chances that their applications will succeed. Fortunately, there is also a growing number of free applications that compete with these commercial ones, and they do a great job (to mention a few: Camino, Adium X, Colloquy, Smultron, etc.).
Even more, given that Mac OS X is based on Unix and that it provides a X Window System server, it is possible to run most of the free applications available under Linux in this operating system. Just check out, for example, The GIMP, or fetch pkgsrc and start building your own favourite programs!
Aside that, there are also very popular commercial applications available for this OS. These include the popular Apple and Adobe applications (iWork, Photoshop, Premiere, etc.) and other such as Microsoft Office, Parallels or Skype (I know, the latter is also available for Linux). It is a fact that nowadays some of these programs are superior to their free alternatives and some people will want to use them. But, ultimately, they have the freedom to make that decision.
In this area I think that Mac OS X is more versatile because it can take advantage of both free applications and some interesting commercial ones. Only time will tell if those will be natively ported to Linux some day or not, but if/when that happens, it will be as versatile as Mac OS X with the advantage of a predominating feeling of developing free software.