The problem with Vista's UAC is that it is a blunt instrument for controlling application installation and activity. With Windows 7, the controls over UAC and the warnings it gives have become much finer-grained, allowing the user to tell UAC that a particular application or process is trusted, and should therefore not be the source of incessant warnings and permission requests. The changes make UAC in Windows 7 a much more useful security technology because it's much less likely to be turned off by the user.
Both Windows 7 and Snow Leopard offer built-in encryption, an important feature if you store personal or company data on a laptop. Snow Leopard has the ability to encrypt your home folder -- the place where virtually all your documents and data files will live unless you explicitly tell them to go somewhere else. It's set up through a simple check box in a system preferences dialog, and it requires relatively little processing overhead after its initial encryption session.
Windows 7 inherits BitLocker drive encryption from Vista with one important update and one significant limitation. The limitation is that BitLocker, which can encrypt an entire disk or any chosen files or folders, is only available in the Ultimate and Enterprise editions of the operating system. The improvement is that, if you have Ultimate or Enterprise installed, you can now encrypt USB thumb drives as you would a hard disk. That's a nontrivial consideration, and one that could drive the selection of a Windows 7 version.
BitLocker can be controlled through AppLocker, a Windows 7 Enterprise feature that permits central control of the applications allowed to run on a system or group of systems. AppLocker is, along with Direct Access (a secure remote access method that can eliminate the need for third-party VPN software), a significant step up in corporate security for Windows 7.
Both Windows 7 and Snow Leopard have built-in firewalls, and each operating system allows for fine control over resource sharing, computer identity display, and network discovery. Many commenters have made the point that Windows 7 security is more advanced and capable than the security built into Snow Leopard. That is almost certainly true. It's equally true that the number of exploits attempted against Windows machines versus Macintosh computers make stronger security a more stringent requirement for the Windows system.
There's another thing I have to mention: system updates. In the time I've had Windows 7 installed, there has been a regular stream of patches and updates. I know that patching vulnerabilities is crucial, and I applaud Microsoft for moving quickly on this front, but the fact that I am regularly informed of updates plants questions in my head over security and reliability. Snow Leopard has been updated, yes, but at far less frequent intervals than Windows 7, a pattern that each operating system is continuing from its predecessor.
Verdict: I'd love to have a clear winner on this point, but in considering everything I'm going to call security a draw. Each system is better than its predecessor. Neither is perfect, but each will allow me to work without constant fear of data loss. It's a good draw, but a draw nonetheless.
Compatibility: Role reversal
In some ways, it's a bit early to draw conclusions about overall compatibility with existing applications, but a couple of statements can be made. First, I have been surprised by the number of Mac applications requiring new versions to work properly with Snow Leopard. For an operating system upgrade that did not carry a new user interface or entirely new code base, the number of bent and broken applications has been large. From Microsoft Office applications through Firefox and even less popular applications, many apps have required new versions in order to function with Snow Leopard.
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