Jaballah,
Your question is similar to asking, "What is more preferred for companies (worldwide) to use: Volkswagen or Mercedes, what are the percentages, and what are their main obvious advantages/disadvantages." Any statements one makes would need to be gross generalities with which others could vehemently disagree.
But, to try and address your question, I shall make some gross generalities (to each of which anyone, including myself, could respond, "Yes, but..."). Generally, companies use Unix-based machines over Windows-based machines for heart-of-the-business, heavily multi-used applications. In organisations with which I work, the companies generally equip their Unix machines with multiple CPUs, more memory, and larger disk arrays when compared to Windows-based machines. Therefore, the Unix-based machines can carry heavier work loads, supporting more users, using more data, and processing at faster speeds when compared to Windows-based servers.
But, generally, Unix-based machines cost incrementally more than Windows-based machines. So, you will often see statistics that the cost-benefit ratios (or cost-per-exectuted-instruction figures) are better for Windows-based servers than for Unix-based machines.
In terms of Oracle processing, generally both platforms are equally stable and reliable.
One issue is that the way you have posed your questions, it is difficult to respond since we do not know that your processing values/goals are. So by-and-large, your questions are "moving targets". Usually, we do not make decisions regarding Unix-versus-Windows-versus-anything-else in a vacuum. We identify processing-performance levels and other topographical requirements first, then identify the ability of various platforms to meet those requirements.
I'm sure that enough information exists on this topic to fill a couple of volumes, but at least these are some general observations that will get you started on your quest.
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Mufasa
(aka Dave of Sandy, Utah, USA @ 16:32 (04Jul04) UTC (aka "GMT" and "Zulu"), 09:32 (04Jul04) Mountain Time)