Sweet potatoes are often touted as a healthier alternative to potato because they have a lower GI. Generally though people eat a mix of foods and it all gets to know it each other in the stomach, so unless you're eating meals that comprise solely of sweet potato or potato, I'm not sure it's relevant, other than – of course – Carbs are Evil. I don't know what it is about nutrition, but it really does generate the most bullshit and woo. The shame is that good, healthy nutrition is very simple thing. There's no money in simplicity, of course.