When you're choosing a language or a platform, the community and available libraries are often more compelling arguments in favor of a technology than your own personal experience. Given solid technology fundamentals, it shouldn't be too hard to learn a new language or a new technology platform. Additionally, you can always hire expertise.
How active is the developer and support community for a language or a platform? This will determine improvements released over time and if there will even be support for your choice in the future. What kind of libraries, plugins, and extensions are available? A large selection of modularized solutions to common problems both saves time and mitigates risk in the long run.
We've chosen Ruby/Rails for these reasons given the active development on the Rails platform, the great developer community, and the huge array of gems/plugins available. This has paid long term dividends over the last few years of development. The fact that Rails serves as a great rapid protoyping and development platform is a nice extra.
The above points assume your product development has achieved reasonable product/market fit. If you haven't validated your fundamental concepts then you should be using whatever tools meet that short term goal the fastest. You're more likely to throw away code until you've reached that level of stability.