One is more important because presumably one is harder and thus more expensive to recover from. Typically the deeper the rot in the foundation, the more expensive the repair.

A data corruption might be also impossible to recover from. You could even only notice it after months of it happening, e.g. if your users spend months collecting data which is only analysed at the end of the year.

You are potentially corrupting ALL of your data, not just a handful. Sure, in the two scenarios you chose to provide (speaking of arbitrary), the JS error is clearly worse. If you take the worst case scenario in both cases, one is much more likely to cause an existential threat to your business.