要するに・つまり: the difference
要するにN2
Meaning
To sum up, In summary, In short
Past papers
Tested once in past papers
Example
要するに何でもいいよね？
In short, it is all the same to you?
つまりN3
Meaning
In short, To sum up, In other words
Past papers
Tested once in past papers
Example
明日何時でも良いんだって。つまり、夜遅くなっても良いらしい。
They said anytime tomorrow is good. To sum up, it seems that even late at night is ok.
How to tell them apart
「要するに」: squeezes a long talk into one key point.
「つまり」: more uses; also rewording: 「父の兄、つまり伯父」 (my father's elder brother, that is, my uncle).
「結局」: the result in the end, after a long time or much back and forth.