If you've borrowed money from family and friends to hire some developers in order to get something working as quickly as possible you should use whatever tools are going to help you make that happen. You probably don't want to be tinkering around with a new language and framework on top of having to work with developers you're unfamiliar with when you're dealing with a project of this importance. Use what you know. If you have reservations about Ruby and Rails start thinking about one of the other languages you've used to get important work done. You can always explore other options once you're making some money and have paid back your initial investors.
Personally, I'd be a little miffed if someone accepted my investment for a specific project and then was using that project as a testbed for some fad web framework for no good reason. These people have placed their trust and money with you. You have a responsibility to deliver now.
At the same time, if I simply had stuck with what I knew over the years I'd still be doing old ASP...i'd be more miffed as an investor if you had a great idea but killed it because you did it in vbscript just because that is what you knew, or if you chose something that would cost a lot more to scale, thus requiring greater investment just to see a return.
Personally, I'd be a little miffed if someone accepted my investment for a specific project and then was using that project as a testbed for some fad web framework for no good reason. These people have placed their trust and money with you. You have a responsibility to deliver now.