Swan says that the two words "mean exactly the same. Till (AmE also til) is informal."
In the third edition (1996) of Fowler's Modern English Usage, Burchfield writes, "It is not true that till is a shortened version of until: in fact, till is the earlier of the two. [...]. The evidence tends to confirm that till is sometimes the informal equivalent of until, but also that in many contexts the two words are simply interchangeable without affecting the stylistic level".