其实哪个都一样,不过否定一般until用的多,来看看他们的关系：
till和until都可以用作连词和介词,用于肯定句和否定句.用于肯定句时,只与持续性动词连用,有一个not...until的形式是固定的表示"到...为止".用于否定句时,通常与瞬间动词连用,也可与持续性动词连用,表示"直到...才"的意思.这两个词一般情况下可以相互换用.例如：
We waited for him until/till six o'clock.我们等他到一直等到六点钟.
The pupils will not begin the meeting till/until their class supervisor comes.学生们等到班主任一到就开始开会.
They didn't talk until/till the interpreter came.直到译员到,他们才交谈.
He did not go to bed until/till he had made sure that nothing was wrong with the pipes.他检查水管,确准它们没有毛病,才去睡觉.
当until用于句首时,一般不能用till代替.在以not开始的从句或短语里,till一般也不可代替until,而且这种句式要用倒装语序.例如：
Until the last minute of the match we kept playing.直到比赛的最后一分钟,我们仍然坚持奋战.
Not until he had finished his work did he go home.他直到把工作做完,才回家去.