Hi Aaron,
I´d say, it depends on what you expect of your allied units. Grabbing objectives, give support, do damage or just harass the enemy?
Dreadspears are very good in holding ground (and especially Objectives) with their +1 to hits if they did not move this turn and another +1 to hit when 20+ models (in total lots of attacks because of 2" range with 2+/4+). With their 5+ Save and re-roll 1´s and 2`s in close combat, they are hard to break.
Darkshards can give you a bit support in your shooting phase. 60 shots on 5+ to hit (4+ when 20+ models) sounds good, but don´t expect too much damage from them. Remember, 16" isn´t too much from your repeater crossbows, as many Units can try to attempt charging them. But if you just need a caddy for your heros and a little support in the shooting phase, Darkshards can be an good option.
I don´t have Bleakswords in my army but in my opinion, they seem to be a kind of a more offensive alternative to the Dreadspears as they generate an extra attack on a hitroll of 6 (5+ when 20+ models). They are good to carry your squishy units into close combat and protect them a bit. Otherway the can do a bit damage and fixate a enemy unit for 1-2 turns.
Anyway, I recommend to build the 30 models in just one unit to ensure you can profit from ther 20+ models ability.