Till is less formal then until. So, in writing, I'd use 'until'. Looking at some of the other answers; there seems to be some confusion. 'Till' is not short for 'until'. It is a word in its own right, which means the same as 'until'. Till is perfectly correct.

proper and improper

‘til is short for "until". so "until" is the correct word, 'til 是一個口語的縮約， 不正式的

http://learnwordlist.com/blog/difference-between-untiltill-and-up-to/2010/10/gre-word-list-test-blog-barrons-vocabulary