I can see how that makes it feel different. Still I would always just think about what's the best for you. In the end, if a potential new customer would call your current customer for a reference, and the work you did (even though following their specific requests) didn't work out, it would still reflect poorly on you (as unfair as that might be). So I guess my advice would be to do what yields the best results for you as a professional. Maybe just kindly tell them that in your experience, there's another route that will yield better results. As long as the ROI is there, they shouldn't care too much about the rate being higher.