Please explain me the use of "Reselect on Service Down" and "Least Connection" and also which is better? Consider me new to this

Which is better? It depends.

Which one is better for you? Without understanding your environment and knowing the reason for your current load imbalance I couldn't say, however if you currently have an imbalance with RR, it can't hurt to try Least Connections to see if evens out the load. But there may be something else at play here....

When you talk about publishers....you aren't running Adobe CQ/AEM are you?