This is a good question. I'm inclined to say that the value proposition for testing is better than for verification. First of all, if your unit is hard to test,

This is a good question. I'm inclined to say that the value proposition for testing is better than for verification. First of all, if your unit is hard to test, it is likely to cause other issues, but probably more importantly, testing finds a lot more bugs per unit effort than formal verification. That last point is based exclusively on my intuition and it could well be wrong--it would be good to hear from people with more experience with both testing and formal verification. Also, now that I think about it, the value proposition isn't fixed for all applications. Testing is the better value proposition if you need to catch the easiest 95% of your bugs (arbitrary number for example purposes), but if you need to catch 99.999% of your bugs, the testing effort may likely be much greater than the formal verification effort. That said, open source projects like Rust get tons of testing for free by virtue of being very, very popular, so I'm not sure how to weigh one against the other.