We used to prefer yams when we could get them, but I don't see them anymore. The reality is that I believe what was called yams were actually a variety of sweet potato. I was told many years ago by a produce professional that yams sold in the us were a sweeter variety of sweet potato. I always thought the darker potatoes were sweeter and I imagine that's just a variety trait.
When we were in AZ we bought white sweet potatoes and I thought they were good with a little less fiber being the difference.
It is no longer possible to get the best varieties of produce as a rule, instead the most productive are available. Tomatoes are probably the best example. Tomatoes in the big markets generally have a texture closer to an apple than a tomatoes so that they are tough enough to stand shipping.