Johnnieb,
I think you're right in the sense that they are both Xanthosoma, although I think the pink, or sometimes purple colored root might be X. violaceum (someone knowledgeable in this genus will know far better than I). Question is...did you get to harvest anything from that great looking specimen?

Johnnieb,
You're right. They do need more time to develop a number of good sized tubers. When I was little I recall my grandmother planting these for harvest and it seemed like the plants were in the ground at least 7-8 months-in the tropics, where there's practically no halt to the growing season.

You can check here
http://www.practicallyedible.com/edible.nsf/pages/malanga

Try a Yuca...Not to be confused with Yucca.
It makes a small tree like bush about 4 feet tall.Yuca fries blow French fries away when the yuca is fresh...Hint...It aint nowhere near fresh in the US Latin markets.Horrible !

Yautia is Xanthosoma violaceum.Good in soups and stews.

I grew up in a farm in the caribbean, malanga is much larger than the yautia and when cooked the malanga is softer than yautia. The malanga plant is much taller and the leaf is much larger that the yautia plant I still eat yautia but do not eat malanga that much. See the pictures, I bought yautia and malanga this weekend.

this is a very old post, but it stil might be useful for reference.
there are 5 malangas and 3 yautias on the list. some malangas are xanthozomas, some colocasias.

Here is a link that might be useful: list of edible tubers with bot names

I make a nice stew with goya canned garganzo beans, carrots and yautia!!

Hi. The malanga plant is Colocasia esculenta. The yautia plant have different varieties. Xanthosoma sagittifolum have white flesh. Xanthosoma atrovirens have yellow flesh and probably is the more nutritious one. Xantosoma violacium have purplish flesh. This last one is my favorite! The malanga is related to the taro of Hawai.

Grocery store? So these are edible? What are they sold as?

Cubans call yautía "malanga", while Puerto Ricans call malanga a whole different root. In the US, I have to look for malanga, when looking for yautía...