两者其实区别不大,你如果一定要鸡蛋里挑骨头,则有这样的解释：
1、从语气上来说,-ing 形式比较personal 和friendly,和 look forward to...相比较少为正式,如果你要表示礼貌而有亲和力则用looking forward to...,look forward to在非常正式严谨的business场合比较适用.
2、国外英语论坛上native speaker给的解释二：
“The difference is how the speaker perceives their own words:
an action seen as a whole/accomplished (I look forward...) or continuing (I am looking...).
一个动作的结束（look forward to...）和一个动作的持续（looking forward to...）
Unless very self-aware or possibly an English teacher,an ordinary native speaker will be unable to explain their choice between the two and so the difference between a mental impression (I look forward) and a mental activity (I'm looking forward).In such cases,attempting to explain these subtleties to your students may prove counterproductive,unless of course they're advanced enough to handle it.It is my policy,that until then they're perfectly fine knowing there is no real difference there.”