Alpha and Beta OS releases are intended for testing and will likely contain bugs. In the past, some of these bugs have been known to destroy some hardware so unless you are using a computer that you don't care about, you may be risking permanent damage to your PC. In addition to that, the bugs in the alpha and beta versions may be such that only the most advanced users of the OS can successfully fix the bug and make the OS operational. It is not recomended to install these on a PC that you intend to use.
Apple Macintosh Leopard has very spotty support for many 'off the shelf' PCs and it is very likely that your hardware would not be supported. If you are buying a new computer that has this OS installed, you will not experience this. If you have your heart set on it, then you will need to read up on it from one of the many "hackintosh" sites. http://lifehacker.com/software/hack-attack/build-a-hackintosh-mac-for-under-800-321913.php.
I can tell you from experience that Vista sucks. It has browser crashing issues, it takes a lot of memory and HD space and it just plain is not cool. Vista, however, is not an alpha or a beta and is not likely to harm your hardware so it is safe.
For the question of which is better, it is really a matter of what you plan on doing with the PC. If you are a hardcore gamer, then it is conventional wisdom that you need a Windows PC to do that. If that is the case, then you should use Vista. It has DirectX, most hardware is supported and it will likely run all of the latest games.
If this is not an issue, Mandriva and Macintosh are both 'unix' like operating systems and they both operate in a similiar manner. I personally use Mandriva 2008.1 and Mandriva 2009.0 and I can recomend it. It is a wonderful - beginner friendly - version of linux and unlike Mac, it's free and most - if not all- of your hardware will work. You should not install the alpha or beta version though and if you are not already a linux user, you may want to use 2008.1 because there is a bug in the 2009.0 version and your install may not boot into a graphical interface. This is one reason that Linux is not really for everyone at this time. Linux will require you to learn and tinker around your PC to some extent. If you want to try a 'unix' like OS and you expect it to just work, then you should use Mac. If you are interested in learning about the nuts and bolts and you want your computer to be endlessly configurable, then Linux is for you. If you have some proprietary software that you 'just can't live without' then you need to make sure that whatever OS that you use will run it.