Full Disclosure: I'm the founder of ChimiChurri (see http://chimi.co). We build the kind of membership website/application you are looking to build by leveraging a proprietary product building platform and a professional services team. I'll attempt to keep my answer as honest / unbiased as possible.
The quick answer is you can likely build your membership website with either. But, WP is, IMHO, the wrong tool for the job. I have built WP sites and coded WP plugins before and, from a software engineering perspective, you'll likely very quickly end up in what I'd typically describe as "plugin mess". Meaning you'd probably end up with a duct-taped ball of WP + custom theme + several (different) third-party plugins + custom code + third party services that will be extremely hard to maintain and continue to iterate and build upon. So, while doable, you'll sacrifice quality and accumulate "technical debt", to say the least.
As per Ruby on Rails, it is a very solid MVC framework I have used to build many products and applications throughout my career, including marketplace and market-network type applications like the one you are describing. But, with RoR alone you'd indeed have to build lots of custom functionality to get to MVP or v1, and that will take money and/or time.
With Chimi (our product building platform), you can get the best of all worlds: a solid, robust Ruby on Rails application that has lots of the features you are looking for available out-of-the-box, without sacrificing quality, flexibility, or ownership, and with a relatively small budget and fast execution and implementation time.
With Churri (our professional services) you also get access to top-quality engineers and developers that can build custom features and integrations unique to your needs.
Let me know if you'd like to setup a time to chat - I'd love to give you a demo and see if we'd be a good fit for your needs.