Short answer:
Ubuntu
Long answer:
Ubuntu doesn't have the many memory leaks (the OS deallocates resources after it closes a program, it not all the resources are deallocated then you have a memory leak, over time this will slow down your machine and force you to reboot) that Windows has as each application is run in isolation from each other and when they close, the resources are returned to the OS (Ubuntu inherited this from Unix).
I find 99% of the time that Ubuntu boots faster, and runs applications faster then on Windows. The only exception to this are applications that are integrated into the OS on Windows (like IE).
This gets me into security, because applications are not integrated into the OS and there is no root/admin account on Ubuntu, it is inherently more secure then Windows (not to mention their are far fewer viruses and malware for Linux then Windows).
I find Ubuntu to be more user friendly then Windows and I find that brand new users to computers can pick up Ubuntu faster then Windows, but since Windows has had greater then 90% market share of all desktop computers since 1995, more people are familiar with Windows and as such will find Windows more user friendly.
I strongly recommend that you download the Ubuntu LiveCD. The LiveCD will allow you to try Ubuntu without having to install it on your machine. Since the LiveCD is free (just costs you 1 CD) you have nothing to lose by trying it out. Some things to watch out for though is that you will not be able to run Windows applications on Ubuntu so you will no longer be able to buy the latest games for you computer if you switch over. There are games for Linux ( http://gaming.gwos.org/doku.php/guides:64bit ) though.