Farne fills a different roll: that of a summoner; Bishops are hybrid mages that turn into healers on skill duration, so they are designed for maps with reduced ranged spots or low unit placement limit so that you can get an AoE magic DPS and a healer when in a pinch
That in mind, bishops have a bit more range than mages so in some cases that range will matter when chosing either class. Also the difference in AT is not that much inho at least