Super Baby Food book says, basically, sweet potatoes are yams...
Choosing " But them loose so you can get a good look at each one. Select small to medium sized, thick, chunky, well-shaped sweet potatoes that taper toward the ends. Skin should be dry, firm, bright, smooth and uniformly colored.... Avoid potatoes with cuts, worm holes or other damage. Decay, found often in sweet potatoes, can be in the form of shriveled ends or wet, soft, discolored, or sunken area in the skin. Cutting out a damaged area does no good. One pbad spot ruins the whole potato and the rest of the potato that still looks good will probably be affected and taste bad."
HTH
~Rachel