Below are several very compelling facts and articles that discuss the enormous differences between zoos and sanctuaries. It's not hard to understand why Tembo & Sunda's best interests would be to retire at The Elephant Sanctuary.
http://rumblingsfrompaws.wordpress.com/2011/12/05/zoo-vs-sanctuary-the-aza-position/
http://www.pawsweb.org/documents/HUTCHINS_RESPONSE.pdf
http://www.elephants.com/newsStory.php?newsID=425
http://www.helpelephants.com/zoos_vs_sanctuaries.html