La version en español. 
When I first arrived in Mexico I was surprised to discover that there were actually two different translations for Sustainability in Spanish-Sostenibilidad and Sustentabilidad. Since my colleagues at UNAM's Institute of Ecology 
used Sostenibilidad I decided to stick to that version for all my work here. Inevitably when I present about Sustainability one of the first questions is if what I was talking about was the same as Sustentabilidad? At every opportunity I asked people working in the field if there was a difference between them, but two years later I have still encountered very few rhymes and even less reasons for a distinction between them.
Searching online is also not particularly helpful but does yield some background. Dra. Aurelia Vargas Valencia from UNAM's Center of Classical Studies found 
that neither Sostenibilidad nor Sustentabilidad are specifically listed in Spain's Royal Academy Dictionary, the official bible of the Spanish language. She believes that both words are interchangeable and acceptable, but if the roots of the words are traced back Sostenibilidad has an older heritage and therefore could be preferred . Others have hypothesized 
that since Sustainability as a concept originated in English, Spanish words were chosen which sounded closest and translators made different initial decisions, leading to the current confusion.
*For a little more etymological discussion see my update on 8/19/2014. 
Recently I realized that I might be able to find a little clarity using two neat tools from Google. One of them, Ngram 
, traces the frequency of word use in all the published books Google has scanned. The other, Google Trends 
, measures the popularity of different terms in web searches.
This graph from Ngram shows that over time Sostenibilidad has gotten mentioned more and more frequently in books and while Sustentabilidad also grew for several decades its use appears to have declined in the last few years. In 2008 Sostenibilidad was used twice as often as Sustentabilidad. Unfortunately this data set only includes books published up to 2008, leaving us with no idea if this trend has continued in the last six years. I next looked at Google Trends, since the data on web searches is current to within a week, the results unfortunately did not clarify anything.
This graph show a huge amount of variation in the frequency of use of both terms with neither being clearly more favored over the other for the period from July 2007 to July 2014. I also produced a trend line for each of the terms which shows the use of Sostenibilidad to be stable while Sustentabilidad has been trending upwards... the opposite of what the Ngram data suggested! Assuming the trend towards Sostenibilidad in books holds up, I would draw a tentative conclusion that Sostenibilidad is more popular in academic and journalistic circles while Sustentabilidad is more common among the general population. And in fact this hypothesis was held up when I searched Google Scholar 
for each term . Sustentabilidad returns 72,400 results while Sostenibilidad returns 230,000, almost 3x as many.
Fortunately there is one more little bit of analysis that we can squeeze out of Google's data, the country of origin of the web searches. From this a very clear pattern emerges.
Wow! Mexico accounts for nearly half of all searches for Sustentabilidad. Sostenibilidad on the other hand is more evenly distributed across a wide range of Spanish speaking countries. Sustentabilidad is used more often in Chile as well as with decreasing frequency in a couple other countries.
So ultimately we can't conclude that either version of Sustainability is overall predominant in Spanish, though Sostenibilidad is clearly dominant in academic publishing and one or the other is in different countries. Whether one uses Sostenibilidad or Sustentabilidad it does not really matter in the big picture but the consequences are bigger than just a semantic debate. The problem is that instead of creating a unified field of research and practice, Sustainability is artificially divided into two camps-at least linguistically-and for outsiders or someone new to the field they may only end up encountering half of whats out there (if they search by only one of the terms).
So what after all this, what am I going to use? The case seems slightly stronger for using Sostenibilidad and if I can begin to win over some of the Sustentabilidad folks here in Mexico we could tip the global scale in one direction and unify Spanish Sustainability.