I'm curious why you would choose Node.js instead today. I've done a little Node.js work, and I would unequivocally use it again for anything that needs a fair amount of concurrency, or high volume, lightweight requests. Other than that though, the only advantage I see over other server side stacks is a slight mental bonus in using the same language on both the client and server? Are there any other major advantages you see over PHP/Python/Ruby,etc. stacks?

I don't think anyone's ever subscribed to a SaaS product because it used X technology over Y. I also think you're way too focused on the tech and the list of features, instead of selling a solution.

You can say all you want about invoicing being an over saturated market, but I launched a very successful project management product a few months ago. Focus on doing a few things very well, and you can do well in just about any "saturated" B2B space.