I would never recommend a Microsoft server unless someone needs to run ASP. There are numerous advantages with Unix/Linux systems, especially when you are working with SEO.
From what I have seen, it doesn't appear that a Microsoft server is more secure than a properly configured Linux server. Over the years I have moved a lot of sites away from Microsoft servers and onto Linux due to site owners frustrations with frequent hacks. The key words here are "properly configured". A lot of hosting companies do not do a very good job with securing their servers.
"It's inexcusable for scientists to torture animals; let them make their experiments on journalists and politicians." -Henrik Ibsen