Which is better PBX or VoIP?

I'm curious to know which communication system is the superior choice between PBX and VoIP. Can you elaborate on the advantages and disadvantages of each, and help me understand which one would be the best fit for a business looking to optimize its telephony capabilities? Specifically, I'm interested in factors such as cost, scalability, flexibility, and reliability. I'm also wondering if there are any industry-specific considerations that might sway the decision one way or the other.