Hadlee has a stupendous bowling record in test wins:
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That's better than anybody else in test cricket history with 100 or more wickets in wins. And he did it over 16 years (longer than Marshall's (12 years eight months) and McGrath's (13 years two months) entire test careers). I concede that you said "for a decade".
In the March 1974 win over Australia, Hadlee took 3-59 and 4-71.
In the March 1990 win over Australia, Hadlee took 5-39 and 2-70.
Do you think that Marshall could have carried New Zealand for 14 years until he was 39 like Hadlee (1976-90) when he was finished at 33 despite terrific support and a significantly lighter workload? I don't. That tells me that Hadlee had something as a bowler which Marshall did not have. And that's a plus in Hadlee's favour. He has a number of metrics over Marshall, it isn't just one-way traffic. In the 1987 Boxing Day test against Australia, here are Hadlee's bowling stats (I believe it was a decent batting track):
44 overs, 5-109 and 31 overs, 5-67
Hadlee was nearly 36 1/2 years old here, over three years older than Marshall was when he retired as a test bowler. And in that first innings, he bowled 44 overs, i.e. 264 balls. Marshall never bowled more than 222 balls in his entire test career (Hadlee's maximum was 344 balls). Hadlee bowled more balls per innings in this test match (225) than Marshall bowled in any single bowling innings in his entire test career.
Hadlee first became the ICC #1 test bowler after his 48th test match (March 1984) and from then until he retired, he played 39 tests. He was ranked #1 after 33 of them and #2 after the other six. Marshall first became the ICC #1 test bowler after his 34th test match (December 1984) and from then until Hadlee retired (so we see the overlap), he played 35 test matches. He was ranked #1 after 12 of them, #2 after 22 of them and #3 after the other one. Hadlee did better than Marshall in the rankings when they were both at their best (I appreciate that it takes the rankings a little time to catch up on the performances and that inactivity can hurt you). After Hadlee's final test, his ranking was #1 with 879 points. Marshall was second with 862 points despite being nearly seven years Hadlee's junior.
I'm not saying that Hadlee was a better bowler or greater bowler (there may be a distinction) than Marshall, I'm really not. But you could make an argument that he was just as you could make an argument the other way. It just depends on what you are looking at and how you weight the various metrics.