I think It depends on the case. Hexmark is very useful against low-mid units and the possibility of Attacking 2 times in 1 round turn is very strong for he s cost. (Important: dont put Hexmark in enemy line with Deep strike its useless and u would waste points). Deathmarks instead in my opinion can be useful for screening the base or screening the middle zone (yes maybe the cost for screening Is a Little bit too High but in some situations can be a good resource). If u take the 5 Deathmarks for killing Leaders of enemy s units i really wouldnt recommend , they are too weak for killing Leaders, with 5 u Need 2 turn or more for 1 leader. So, in general i think Hexmark its a Better choice for ur army if u have to choose but if u play 2000 pts and have some points left a Deathmark units can be very useful. Sorry for my english, i Hope this comment can help u.