J.B. Lightfoot, in his first volume on Clement, writes:

If we had to describe briefly the respective provinces of the three great Apostolic Fathers, we might say that it was the work of Clement to co-ordinate the different elements of Christian teaching as left by the Apostles; and of Ignatius to consolidate the structure of ecclesiastical polity as sketched out by them; while for Polycarp, whose active career was just beginning as theirs ended, and who lived on for some half century after their deaths, was reserved the task of handing down unimpaired to a later generation the Apostolic doctrine and order thus co-ordinated and consolidated by his elder contemporaries--a task for which he was eminently fitted by his passive and receptive character. (Lightfoot, p. 8)

I'm not so sure it is that neat and tidy--for example, though Ignatius makes many comments about the roles of bishop/elder/overseer/deacons/etc., I'm not convinced that his letters can be interpreted as a manual on ecclesial polity--but the quote does highlight some major emphases of the respective authors.