As far as I see Linux hosting has been more powerful for years. But the thing is that windows hosting is getting more popular nowadays. I hope we will start selling windows vps soon as there is high demand on it.
I prefer to offer linux because i did not see good demand of windows hosting and yes ofcorse windows hosting is also a source of profit and source of new clients. If You have some extra bucks then it's okay you may offer windows too.
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Linux generally has lower costs as it uses less resources and is free but Microsoft may open you up to additional customers who require ASP support. I guess it is upto the individual host and their business plan on whether they offer one or the other or both.