It doesn't matter, if you are a competent programmer you can use .NET, RoR, Python, Java, PHP or anything to make it work efficiently. Tools do not make the system better, the people working on it do. There is 'magic' in both the systems that you mention but you should be aware of the underlying 'magic' and architect a solution that works best. Rarely today are programmers limited by platform. The only concern with .NET is platform lock-in but if you go with mono there is no concern.

You will find languages and platforms are like religions or brands, everyone has their favorite because it worked for them but it doesn't make them the best. Just the best for them and their conditions/constraints/knowledge.