Two different words for the same thing.
Hassock is far the older word. According to the New English Dictionary a hassock is a “firm clump or tuft of matted vegetation; esp. of coarse grass or sedge, such as occurs in boggy ground; a ‘tussock’”.
The Vocabulary of East Anglia, compiled by the Rev Robert Forby and published in 1830, describes how “hassocks in bogs were formerly taken up … shaped, trimmed and dressed …to make kneeling much easier than on the pavement of the church”.
The New English Dictionary also declares that “Hassocks of turf or peat, formerly used in the church, are still (1897) preserved at Lower Gravenhurst in Bedfordshire.” Are they still? Does anyone out there know?
The word Kneeler came into use during the nineteenth century. According to the New English Dictionary, a kneeler is “a board, stool or hassock on which to kneel”. The first example it quotes is from John Newman, who in “Loss and Gain”, 1848, referred to pews with kneelers.
More about the history of the Kneeler.