You want to start with something you know, especially at a time when getting an MVP, making iterations, and being nimble is important. Because you both already know PHP, this is a no-brainer. Things have changed dramatically with every aspect of web development in the past 6 years, and PHP is the thing that (although its changed a lot) has probably changed the least compared to your web front-end (HTML5/CSS3), client-side JS, and responsive design (which basically didn't exist 6 years ago).
You should also consider the economics of your situation. Developers who know enough about Go and Node.js to do it professionally are going to charge you much more than those who code PHP professionally.
Ultimately, you're probably doomed to failure just by raising this question. Its been stated ad nauseam on HN before, but you need to iterate quickly and discover what works and drives traffic/sales. Don't waste time trying to learn a new programming language. If you want to devote any time to learning something new, it should be learning how to market your product better.