"Chamber's 20th Century Dictionary", 1981 reprint and "Chamber's Combined Dictionary & Thesaurus" 1997 edition, both give the two forms, "till" and "until", as totally synonymous.
Convention has probably rendered "till" a less formal form of the word, but both are linguistically and gramatically correct.
For this particular text, if the final use is "formal", prefer the form "until" (for the conventional reasons above), if not, the form "till" is perfectly acceptable