Interpolation vs ngram-merge

[marco turchi]

Dear experts,
i have a question for u.
I have two dataset, and I want to construct a LM that contains both the dataset.
srilm provides me two different paths:
1)to create 2 different LMs and then  interpolate them
2)to count the n-gram for each dataset, merge these counts using
ngram-merge, and at the end construct the final LM.
which are the differences of these methods?
Can u suggest me a paper or book where I can understand these differences?

Thanks a lot
Marco