You will end up throwing out much of the initial code, but use what you know in order to get in front of customers and collect feedback.
Your customers don't care what language/framework is being used, they only care that you are easing a pain point for them.
I know of two people who started web based startups one java and the other PHP and asked the same question. Both ended up choosing a new technology and regretted the decision.
Learning a new technology isnt just about writing the code to get a working prototype or finished product as quickly and easily as possible. Its about the learning about the things that a particular programming language cannot do and its short comings.
You may want to consider Play framework. another modern web framework that uses scala - which is actually an enhanced and more productive java (and if I recall correctly can compile plain old java as well.. )
I'm using it within my company and for my personal projects. It's very easy to get started.
Lastly I'm of the feeling that learning a new language is going to really impact your ability to put best development practices in place as you will be working to overcome 'basic' hurdles like learning the system API, stack, conversions, etc...
RoR/Django is good for prototyping and for fast start, but if you don't know it's it no sense I guess, you will make it more faster with Play/Scala.