By the time I shipped a new version, the number of concurrent visitors for my app dropped to a lower level; so I can’t make a conclusive comparison between two stacks. But I don’t need an experiment to argue that having websockets is more resource constraining.
I think it’s important to think about the business aspect. If you are already heavily invested in one framework, does the benefit outweigh the cost of rewriting the app? You have to also think about the opportunity cost of learning a new technology.
SEO can be done with Meteor. Flow Router has SSR (although not 100% yet), and Galaxy has prerender built in (so I heard). For React + Redux app, SEO is trivial because of easy server side rendering.