For the nightshade intolerant, avoid the white potatoes. Also, most potatoes are doused in herbicides and pesticides while grown, then spayed with mold inhibitors and sterilizers before being shipped. The people who grow them admit they'd never eat them. Organic would be the only way to go. But why bother, if you've got sweet potatoes?!
Sweet potatoes, on the other hand, are not a nightshade and make a great staple starch, especially if organic.