Ubuntu vs Windows 7
 Infection Possibility
Since Windows is sold with almost every PC, it is logically the number one operating system operated by most consumers. The result is that more malware, viruses, and Trojans are developed for Windows. Linux users are usually much savvier when it comes to general computer use.
Linux has been used on servers for a near decade and was a target to very sophisticated malware and a lot was learned from this constant beating; Linux security is very tried-and-true nature. The source of Linux is open and shared, making collective patching and inspection as easy as it gets and allowing 2 hours reaction speed between detection of malware and release of a patch (see Debian). Windows users will have to rely on antivirus updates to keep their machine somewhat protected, until Microsoft or a responsible company release a patch (if ever). This is also one of the reasons that desktop Linux-based OS, such as Ubuntu, usually does not need any firewalling.
Antivirus companies do not exist without hackers producing malware. There has been speculation that Windows security is intentionally low, and that antivirus companies promote this supposed lack of security to better suit their own agendas. Of course, Ubuntu (and all Linux-based operating systems) is not immune to computer viruses, as is evidenced by the implementation of such free Antivirus software programs as ClamAV. Ubuntu does offer the advantage of having far fewer viruses and malware that can run on it. This is mainly due to two reasons: 1) Ubuntu is a Linux distribution, which inherently has enhanced security measures, and 2) There are far fewer users operating Linux-based operating systems. Naturally, hackers will target the operating systems that have the largest user base. The main short-coming for most average Linux users is that they do not run any antivirus software at all - either paid or otherwise. This in itself could increase the chances of those said users to be able to harbor or become victims of malware - or both. Additionally, as the Linux user base increases, the community could potentially see an increase in malware available for Linux-based platforms.
Antivirus software relies mostly on heuristic methods to determine whether the software in question is dangerous. These methods are highly unreliable. They also use virtual machines and virtual execution and analysis before actual execution, this however, is no-saver as modern malware can obfuscate, mutate and update itself on its own.
Linux has very deep, yet simple and efficient file rights management out of the box and additional restriction mechanisms available. Windows started implementing the same technique starting with Windows 2000, but even today its security model is weak.
Linux has "ability to be executed" as access parameter and utilizes NX (no execute) together with memory address randomization (where available) very efficiently, where Windows relies only on NX and file MIME types to determine if file is executive (very often mistaking and executing "pictures"! See 2010-3970 as the most recent entry).
Ubuntu software is updated centrally and automatically, making the application of security patches centralized. In the past, Windows users were required to search for and install the latest security patches themselves. There have, however, been improvements in this area as Windows Update was redesigned for Windows Vista and Windows 7. Many third-party software vendors have also increased their update features, but this still leaves a large margin for error, as Windows updates and third-party software updates cannot be managed from one centralized location. Due to the central and open nature of FOSS development, it is very unlikely any malware can make it into the repository. Be aware however, that closed source software, which is also available for Linux (such as Adobe Flash) has a long history of un-patched vulnerabilities. Updating both OS's and using extensions, such as NoScript is useful for both.
The Linux kernel binary interface is more unstable than that of Windows. This leads to situation, where with change of kernel compiler, system C library and its version - all binaries must be recompiled or they will break. Windows however has been trying to maintain compatibility with older revisions and APIs for years. Because of this, infecting Windows by exploiting errors is MUCH easier. Windows also has a LOT more old crap and bloat in form of outdated libraries. Still, the compatibility with legacy on Windows is not that great (but its non-existent on Linux, either application is maintained or it will eventually disappear because of breaking).
Windows software usually costs much more than of Linux equivalents. As such, many people illegally download Windows software and many crackers integrate malware deeply within their releases, making such Windows machines a "zombie" (part of botnet) as soon as such software is installed.
 Software Choice
Because Windows has been primary desktop platform for years, Windows has the most software available. However, if only the newest or most up-to-date software is considered, the value will be lower, but still will beat Ubuntu, Gaming was one that areas, but now this situation change a lot with Steam for Linux.
The most drastic difference was in Gaming, where Microsoft has been pushing and polishing its DirectX exactly for the purpose of games to relate on its Windows-only technology than on feature-comparable crossplatform OpenGL. This is however again changing, due to OpenGL availability for smartphones(android), consoles (ps3) and Macs. Apart from it, the project exists to make free implementation of Windows API available on Linux and Unix systems making running of Windows-only software at near native speed possible (See WINE). Ubuntu is OpenGL capable, has increasing number of games and is capable to run at least 2/3 of Windows-only titles via WINE, 1/2 of them perfectly. Sometimes even better than on Windows. With Steam for Linux, chipmakers also start to build better graphics drivers for Linux, since Valve plan to launch a SteamBox using Linux.
As application development platform however Linux software choice is larger than that of Windows.
Overall amount of software for Ubuntu should be more than enough for its use as Desktop OS. Most of software, unlike on Windows, is free and usually opensource.
Ubuntu does not need its drives defragmented (except using non-extX based FS), it does not need any special tuning, software installation is done very quickly and from one place, its kept on current status automatically, the system overall is very predictable.
Windows does require more maintenance time and much more setup time.