This essay has been referenced over and over here, but have yet another link: http://paulgraham.com/avg.html Check out the whole thing, the money quote for this context is: "Back in 1995, we knew something that I don't think our competitors understood, and few understand even now: when you're writing software that only has to run on your own servers, you can use any language you want."

Contrary to the other comment, there are business advantages to using a particular language. (Though I would concede if your only choices are Ruby's Rails, Python's Django, and PHP's Yii, for many problems there isn't much to compel you to one or the other besides your preference and available talent.) I could write a large comment going over the pros/cons for different use-cases of PHP (plain or with a framework like Yii), Python (with Flask), Java (with enunciate), and Node.JS (those are the only languages/environments I've built larger-than-toy webapps with; I still need an excuse to use Clojure's Noir for something). My list would not just be language-war pros/cons but business value considerations and hypothetical consequences. It's not the most important choice you can make, but it should be considered if there's more than one option because the type of problem you're solving can be made much simpler or easier with the right tool.

I think you've got the right mindset, and that's to use what you know for anything important you need to finish soon, but I'd recommend checking out the other environments on your own time just for fun. Also before starting a project, research to see if it's a solved (or mostly solved) problem for another framework/language. Even if there's a learning cost to something brand new, depending on the problem it can be well worth it for the overall cost reduction that the tool provides. There are a lot of "We learned and used/migrated to Erlang" stories out there because Erlang solves particular problems very well.

I would be interested in reading a blog post which details your experiences and business decisions behind all of the frameworks you have mentioned.

I often find myself wanting to start new projects in $new_platform but have no idea how to really compare them before I get started, save for looking at their documentation or advertised features.

There is no business advantages at using Python/Rails over PHP. (and vice versa)

You should only use the technology that suits your company the best (in terms of needs and knowledge -never EVER start a company based on a tech you don't understand or know)

Depends. The availability of open-source libraries in a language can be a big advantage.