There isn't much difference between the two words, only the difference on the tone."谢谢" is to "谢了" what "thank you" is to "thanks". My English is not very good, also the explain may be not very clear. But I hope my answer can help you.

"謝謝" 究竟是甚麼意思? 還得從 古文 和 方言 考證起 請見 http://www.italki.com/entry/374837 說 "謝謝" 的古義 其實 是 莊重的 含有歉意的 less polite 若翻譯成 "不太有禮貌" 事實上 是與母語人士的語感是有差距的 我會說 "謝了" 是 "較為隨意" 的說法 用於關係較親近的朋友之間 台灣人會說成 "謝啦" "謝了" 為甚麼奇怪? 因為 "謝了" 諧音 "洩了" 成為男性隱晦語

No difference in meaning. "谢谢" is more formal and polite, while "谢了" is commonly used between close friends. It is ok to use "谢谢" all the time without mistakes. For "谢了", I think it is safer to use to somebody who say this to you first.

谢了 is less polite