I will属于临时决定.而I'm leaving属于事前打算好的.
比如如下场景：
1.用I will
A:When are you visiting Mary?你打算什么时候去看玛丽啊?
B:I'm planning to visit her tomorrow.我计划明天去呢!
A:But i heard that she is leaving for Beijing this afternoon!可我听说她今天下午就动身去北京了
B:Oh,my god!I will go right now.I 'll leave in 10 minutes!啊,天啊!那我现在就动身,十分钟后就出门!
（临时决定）
2.用I'leaving
A:When are you visiting Mary?你打算什么时候去看玛丽啊?
B:I'm leaving in 10 minutes!我计划十分钟后就去.
（计划好的）