React.js at the present time. When Angular 2.0 is released definitely check it out, but I wouldn’t start new projects with Angular 1.3 at this time as it won’t be easy to migrate to 2.0 (complete framework rewrite). React is solid, simple and widely used in production by Facebook.
When you start a business you know nothing. In fact you know less then nothing about your target market since you have your assumptions which might take you in the wrong direction. And there are lots of technology-savvy advisors selling you their technology taking your time which you should spend on your business.
It depends on the scale of your operations, but in most cases – yes. It’s the best approach as you don’t have to deal with technical stuff and you can focus only on the business side. Remember time = money, maybe you will pay more but you will get more time instead.
Some Rails dev shops used Backbone.js with Marionette in the past for their simplicity and well working with apps which aren’t SPAs (most of the projects aren’t).
As of late 2014 there’s a trend to use React for this kind of tasks which I highly recommend – it’s simple, powerful, fast and can be used on the server side too so it won’t hurt SEO, which is great advantage over AngularJS or Ember.js. For larger apps use the Flux pattern and you’re set.