Snow Leopard only works on Apple computers which are Intel Based and only a few specific options (unless you hack the os which cause instability in a lot of cases). On the other hand windows 7 and Linux destributions work on any pc out there. If apple releases their OS for all pc computers than I would consider using Mac Os more.
Stability on their respective systems is the same. Windows 7 has improved with drivers/features/appearance to the point that I prefer it more than Snow Leopard, although in the end it is more of a personal choice.
Price wise windows 7 is cheaper when looking at the whole picture. If you have a non-intel apple system you will have to purchase a new system all together (which will include in the price the full leopard software). So anyone that has an older system will not be able to upgrade. To buy an apple system at this point with snow leopard, it costs more then buying the same hardware pc with full windows 7 ultimate. I assembled an intel i7 920 with 6 gb of ram and a 1 TB of hdd and etc for around 1000 CN dollars. So with windows 7 ultimate it will go up to 1400 CN dollars which is still less then an apple desktop computer (which at the moment they do not have an i7 desktop yet, I believe they will be releasing it with snow leopard os).
For people that have the leopard it is really cheap to upgrade but for the rest it will cost a lot more than those that are upgrading a windows system.
My personal opinion on the OS: #1 Windows 7 RC is extremely stable and I love the interface. Pricey but when compared to Snow Leopard cheaper (unless you have an apple system with INTEL to upgrade).
#2 Ubuntu, stable fast and nice to work with. If it wasn't for the messy file structure (no defined rules so you get a lot of software installing in different folders) it would be #1 since it is free.
#3 Snow Leopard nice OS, easy to work with, and a nice software package for video and graphic's enthusiasists.
For new users interested in graphic applications Mac OS would be best since it comes with them and they are simple and easy to use.
For power users, programmers and etc Linux systems.
For heavy office usage, gamers and etc Windows 7.
Oh and virus wise they are all the same. Difference is that 85% of the world is windows based so when it comes to viruses it doesn't make sense to create one that only infects less then 15 % of the worlds computers. That is why windows has more viruses. As a programmer I hate when I hear Linux or Mac OS is better because it doesn't require a virus scanner. All programs created have security holes, since we are talking about millions lines of code created by human beings. So none of the systems are more secure than the other one.