My first rule of computer purchasing:
Application software (i.e. the software you use to actually accomplish a task) has real value. Operating systems and hardware are merely the platforms upon which you run applications software. There is very little point in having the "best" platform (whether you consider Mac, Windows, Linux, etc to be the "best"), if you can't afford the applications software you need, or worse still, if it simply doesn't exist on the particular platform.
My advice:
1) Choose your software first. Make a list of the types of software applications you wan to run - e.g. word processing, spreadsheet, CAD, FEA, CFD, etc. Extend this list to give the names of specific packages that you know of which will meet your objectives. List the features that are "must haves" and "nice to haves". Get pricing information, and compare with your budget. Decide exactly which software systems you will be buying.
2) From the list of applications you have decided on, what hardware and operating systems are supported? Can they all run on the same platform? If not, you need to decide whether you will need two operating systems (maybe even two computers - not a great option for most individuals!), or can you live with an alternative software solution on a single operating system?
3) If the software solutions you have selected only run on Windows (say), then your choice is easy - you need a Windows machine. If they are available on multiple platforms, you need to do some more research on performance, cost of purchase, total cost of ownership of the alternatives, etc.
If you only need basic word processing, spreadsheets, etc, then any platform will work fine. If you must have AutoCAD for example, you will need Windows. If you need Pro Engineer (say), you can look at several options, including Unix, Linux and Windows.
Hope this helps!