Cookie persistence: URL or Domain: which is optimized for LB management?

if one application fails the user interface is impacted in that only one portion of the application is available. that's why sticking the user to the same server makes sense. If the server is not working as expected the whole application will be down and this can be explained and managed better than half of the app is working because the app is split on three servers and you need to know which one is not working worrectly.