SCJ Broad : 20 Tests, 58 wkts @ 37.62 (SR 68.67, BB 6/91, 2x5wi)
IE O'Brien : 17 Tests, 52 wkts @ 31.94 (SR 60.31, BB 6/75, 1x5wi)
Broad is currently ranked 24th, his best ranking and his rating is 538 which is also his best. The absolute lowest his average has been is 37.50 after the 1st innings vs West Indies at the Riverside with 3/62
Reliance Mobile ICC Player Rankings
Interestingly O'Brien is ranked one place below, his highest rating is 543. After a wretched start to his career, he pulled his average down as low as 26.64 after his BB of 6/75 at McLean Park.
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On the face of it O'Brien has a decent bowling average, Broad has yet to get his down to the low 30s. Both have played four Tests against Australia, but O'Brien has also played four against Bangladesh. So 35 of Broad's wickets came against windies and New Zealand compared to 23 of O'Brien's against Bangladesh and windies. According to their career best rankings there is little in it, I'd go with the bowler with the average in the low 30s simply because an average of 37 is that of a very ordinary Test bowler, a modest all-rounder, good for a batsman who bowls a bit/lot and frankly should have been dropped before his 6/91 which mostly came after the game was over as a contest. Broad's other 5wi came in an innings defeat as well, O'Brien's came in a game the kiwis couldn't quite force a win from although four of those wickets were tailenders - he did take two of the first four wickets to help reduce the windies to 74/4 before Chanderpaul and Nash rebuilt the innings, not disimilar to how the aussie 1st innings went.
West Indies 307 & 375
New Zealand 371 & 220/5w