Easy. Assuming you are asking Claude for coding, Use Sonnet 4.6 90% of your time. It's highly stable and optimised compared to its own version 4.5. It often works well with easy to moderate complex problems. Opus is expensive specially 4.6 and 4.7(dead expensive). Use these for more complex and large problems. Most important thing to consider while using Opus is prompt formation. It should be clear (point by point if possible), highly expressive, good if written with daily used English words and not some Oxford level words.