I disagree, security should not be your #1 concern when choosing a language. Almost all your mainstream languages can be security, Ruby on Rails, Java, ASP.NET, PHP. I believe your main concerns when choosing a language are:

There are a couple of questions you need to ask yourself:

Business Perspective
Will it be hard to find more developers to work on it? (If you look at job search sites you will notice that there aren't many Ruby on Rails jobs, however there are plenty of ASP.NET jobs.

How important is cost in relation to maintainability/testability? (I know from real world experience that ASP.NET Webforms is a lot easier for developers to understand and development time is faster (therefore you can get a junior developer later down the track), on the other hand ASP.NET MVC allows for great maintainability and testability, however this does require people who are more experienced and know a thing or two about architecture (usually these developers cost more).

Software Cost? Do you have money to pay for additional software. ASP.NET has a lot of tools out there you can pay for, SQL Server, Windows Server, Telerik Controls etc.

Hardware Cost? What type of infrastructure will you need, how scalable should your site be?

How easy is it to secure? Even though most languages have the ability to be secure, other are easier to secure. Take for example ASP.NET, with that most of the time you will use an ORM which usually abstracts the security / database aspect away from the coding.

My opinion
My personal preference when it comes to choosing a web development language is ASP.NET MVC for the follow reasons:

It is really testable/maintainable - TFD/TDD (Test Driven Development)
It demands a good architecture - Once again TDD is a contributor to this as it makes you think about how to architecture the software
Scalable - Like many other languages ASP.NET is very scalable
Developers - There are a lot of ASP.NET developers out there (a lot of competition for hiring them too), however in my personal opinion, I find that ASP.NET developers have higher standards than your average PHP developer.
Familiarity - I'm an 8am - 5 pm ASP.NET Webforms developer
Security - It does a good job of giving you the necessary tools to make the application secure (a lot of security holes with php sites come from simple human error of forgetting to escape strings).

Could you PM me your skype so we can discuss this at some stage?