http://www.alc.co.jp/jpn/article/faq/04/28.html

http://ameblo.jp/be-thechange/entry-11496360789.html

Briefly, you can use both of them when you describe others skill/performance but normally we don't use 上手 when we describe own skills/performance. 

得意; describe the ability of providing something 

上手; describe evaluation based on some specific abilities which can provide something really superb.