Ask HN: Software Stack for WebApp Python vs. Ruby. or .NET/C#

I have been seriously out of the hard core tech stuff for some time. I started as a Perl, then LAMP and Java programmer and then did a couple of I have been seriously out of the hard core tech stuff for some time. I started as a Perl, then LAMP and Java programmer and then did a couple of .NET desktop Applications, but this was quite a bit ago (in tech years)

Now, I have a business idea and I think it has some scope, after talking to quite a few potential customers for requirements, needs etc. I have mapped it out on paper and have a "plan" but I am at a point now where I need to decide on the tech stack. So thought of asking my fellow people here. I have a fairly technical background but have been on the "business side" for some time, so I know the requirements well but need help !!

It will be primarily a Web based App ("SaaS/cloud??"), will have mobile apps (both for Android and iOS) for consuming the data/reports etc. What I am having a tough time figuring out is what technology stack to use. I have been reading quite a bit about Python, Ruby etc. but the more I read the more I get confused.

Can you please recommend what are the benefits of using Python vs Ruby vs old school-php with Postgres/NoSQL or on the other hand there's the .NET/C# along with SQL Azure etc. (and also the BizSpark)?

What will be a good place to start re-learning about how to choose a stack? I don't need hand holding but am just looking for directions to help me figure out ! Any help/links will be really appreciated. If you think the information is too vague please ask away and I will be happy to provide more semi-technical information of what I know :)