Whether Snow Leopard is faster than Leopard seems to depend both on the task and on the machine. In Macworld tests, the biggest consistent speed improvements were in system shutdowns and initial Time Machine backups. Snow Leopard was no faster than Leopard at starting up or running a Photoshop script, and it was slower at duplicating a 1GB file in the Finder and at waking from sleep. Results for other tasks were inconsistent across an iMac, a MacBook Pro, and a Mac Pro.
In short, Leopard users may or may not notice the additional oomph provided by Snow Leopard's 64-bit Finder, QuickTime X, or Safari 4 browser. Further, the effects of two other behind-the-scenes innovations won't be felt until application programmers begin to take advantage of them. Snow Leopard's Grand Central Dispatch will allow programmers to optimize application performance on multicore processors, while OpenCL will let them tap certain graphics processors to give the CPU a helping hand.
The good news today, however, is that both operating systems snap to attention relatively quickly. In my startup tests, Windows 7 required 2 minutes, 17.9 seconds to move from power-button press to ready. Snow Leopard moved from button press to ready in 1 minute, 15.1 seconds. Naturally, I recorded these times was with no password required; password protection will cost you a few more seconds, but I consider it a good investment.
While the two systems weren't identically configured in terms of the applications that load on startup, they were configured similarly. Each loaded a firewall, antivirus software, Dropbox (a cloud file storage system), Plaxo (cloud contact manager), and the Evernote clipper. Each started a network and Bluetooth service set. Each had a healthy number of fonts available and the same set of three printers on the network. In short, they were loaded with software that users might regularly use to get their jobs done.
In general responsiveness, for doing things like launching applications and switching tasks, I couldn't say that one was any faster than the other. I found the two to be very similar. There is one area, though, in which Snow Leopard seemed to perform better than Windows 7, and that's in continous operation over the course of several days.
Here's what I mean: I'm forever shutting the lid on my laptop, picking it up, and heading to another location. If I don't think about it, I can go several days without shutting the system down. Each of the systems does, eventually, need a reboot to keep everything happy, but Snow Leopard can easily go four or five days without requiring a full power-off and cold boot. Windows 7 seems happiest when I reboot it every couple of days, though that's a major improvement over Vista, which wanted to be shut down every night. I understand that my way of operating probably isn't best, but it's also not uncommon, and Snow Leopard seems to enforce garbage collection and polite behavior of applications a bit better than Windows 7.
Verdict: Neither operating system seemed more responsive or faster than the other. There's a clear winner in boot speed, as Snow Leopard is ready a full minute faster than Windows 7. In terms of ability to handle continuous operation, while Windows 7 is a dramatic improvement over Vista, I still have to give the nod to Snow Leopard.
Security: Fortress vs. outpost Windows 7 has made several important strides in security over Vista. The most important centers around improvements in User Account Control, the security feature most responsible for users' profanity-laced tirades against Vista.
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.