It is as if Tove Jansson altered the book slightly for a reprint. Though each passage is very similar, with most sentences being identical, sometimes there are additions or subtractions. So it is not just a different translation: the original Finnish manuscript was changed.

Some people seem to prefer "The Exploits of Moominpappa" over "Moominpappa's Memoirs". "Moominpappa's Memoirs" was actually published in Finnish in 1968, long after "The Exploits of Moominpappa" (1952), so it seems that "Moominpappa's Memoirs" was the final version by Tove Jansson.

I say, buy them both. "Moominpappa's Memoirs" has a very short "Prologue" which "The Exploits of Moominpappa" (ISBN 0140303235) does not have.

But "The Exploits of Moominpappa" (ISBN 0140303235) has a "Gallery" at the end with a sentence about each character, with an illustration each.

For these very small differences, and the differences between the whole books, complete your collection and see for yourself. Since both books are quite cheap and quite short, I feel I might as well.