Your question perhaps should be the other way around - what is the similarity? That is because 'in a long time' is only used in limited contexts. With 'seen' and 'used' you can use either. But there are many cases where you can't use 'in a long time'. You can't say 'the tap has been dripping in a long time'. You can't say 'the house has been empty in a long time'. I think that perhaps you can only use 'in' with past participles of verbs.

前者指在很长的时间里，注重时间过程。后者指很长的时间，注重整个时间段，注重时间的整体感。

They are pretty interchangeable, I can't think of specific ways in which they might not be. If you type a few sentences someone can let you know if the wording comes out awkward.