I use both. Sonnet is faster and more cost efficient. It's great for coding. Where Opus is noticeably better is in analysis. It surpasses Sonnet for debugging, finding patterns in data, creativity and analysis in general. It doesn't make a lot of sense to use Opus exclusively unless you're on a max20 plan and not hitting limits. Using Opus for design and troubleshooting and Sonnet for everything else is a good way to go.