Which one of these two technologies is better for new web development company (Node.js or WordPress)?

I'm not sure there's any correlation between those two choices. Nonetheless, I would definitely recommend the first stack...except you're lacking a real back-end language. Although Node.js can run back-end, it's not the optimal solution and I definitely wouldn't begin learning Node as the basis for my back-end knowledge. That said, I'd highly recommend you learn Python! It's a very simple language that is extremely useful and is great for back-end (you can learn Bottle/Flask to build some pretty nifty server-side functionality). If you're thinking about a web development company, you always have to ask yourself who the target customer is...if you want long-lasting clients where you're building them a huge project with tons of components and functionalities (not a static site), sure, React might be a good choice. If you're building a static site for your customer, and you don't want the customer harassing you every other day after having finished the project, you might want to use Wordpress to simply wrap the project up and say, "Here's the site. You can edit it yourself. I'm out." Food for thought.

As a side note, Python is a TON easier to install on a local computer than PHP. Python is like three clicks and BOOM! you are ready to start. PHP involves virtual machines, the command line, lots of "googling", and some tougher upkeep.