Both front & back. Both have their benfits.When the shoulder girdle is warmed up, supple and flexable enought, than behind the neck pulldowns can be handled without fear. Same factor is involved when doing chin's behind the neck, which is not only a good lat exercise but also affect the posterior delts strongly. As with either exercise, I touch the base of the neck and than lower back to the starting position. Some will hold the grip way too wide, which can over extrend the shoulder joint(s) into a position of weakness (injury). Forming roughtly a 90 degree angle, when touching the neck area, offers a better inline position for the shoulders and also better muscle recuitment of the lats.
Might want to SS front pulls(medium curl grip... if you like) with behind the neck pulls in a workout. Or what has worked for a lot of BB'ers is a Tri set of Straight arm Pullovers, front and than rear lat pull downs. Of course those would be the only exercises in a single lat workout. Good Luck.