Like many have said it is all a personal preference. You will get reasons from Mac users why OSX is better and you will get reasons from Linux users why Ubuntu is better......both are great Operating Systems, but in the end it is just a personal preference.
And like others have also said I think the best way to figure out which you like better is to dual boot. Try both the GNOME and KDE environments too.
I personally have been using Linux/Ubuntu for the last 2 years and have not found anything that I can not do that I absolutely need. I love the ability to customize everything.
In the end the reason I prefer Linux/Ubuntu is because I like the FOSS ideology and the people/community behind it. I can find a free Open Source application for almost everything, and if I have an issue I have thousands of Linux users who have probably already come across it and know how to solve it. The community has also put together a great idea-brainstorming site called Ubuntu Brainstorm to better improve the OS for the users. Oh and Ubuntu 8.04 comes out in just a few days!