You might consider flipping the question...

Pick a metric: (new user signups, weekly users, whatever) Which will help improve that metric? improved UX/I or more features/better code base.

Back it up with analytics/funnel analysis. If you spend time on your UX/I, and your signups increase, keep doing it. If your increase in signups start to flatten after X iterations, think about another way, such as SEO or coding more features that new customers are asking for.