Ask HN: When to re-invest vs when to build new

I have a few iOS and web software products that make decent money, but none (yet) that make enough for me to stop focusing on everything else and work on them exclusively.

I can't decide whether to invest more in the current products (more features, better design, etc), or build something new and hope it does just as well.

For example: 10 products at $500/month = 5k

vs

1 product at 5k/month = 5k

I'm curious - how do you decide when to re-invest in a product to make it better, or when to let it ride and build something new?