How much do you want to know? It's not easy to explain in a few sentences. Search the web, pick up any ASIC design books, you will see you answer. Basically Elmore is simple, but less accuracy, AWE is more complicated but more accurate. There are always some trade off between accuracy and complexity in modelling. Here is a documrnt to get you a fast start https://www.ece.gatech.edu/research/labs/gsigroup/publications/azeez_idm.pdf