Could go either way really. Hadlee is perhaps my favourite bowler of all time. I love his action, his style and his reliability. His record of taking 5 wickets a test throughout his career as a pace bowler is one that's only exceeded by Lillee in the modern era, and he was a genuine matchwinner.
McGrath is really his equal in just about every category though. Hadlee took more wickets, but had the benefit of relatively little competition, while McGrath excelled and stood out even in a team containing Warne, Gillespie and company. McGrath also played in an era less conducive to fast bowling, and arguably faced a greater array of top class batsmen in his era as well. He can be unerringly consistent on the worst sort of road, and utterly unplayable when he gets a bit of help. Hadlee could bat, and is therefore the better player all-round, but I'll go for McGrath since it's a bowling comeptition alone, and I've had more chance to appreciate his brilliance.
Both of them are among 3 or 4 who could be deserved winners of this competition IMO.