At level 10 you simply take what ultimate you prefer playing with.

Ultimately at Lvl 16 you'd take Horadric Staff for cc-deckard as it provides cc, and a nice niché stun, problem is it requires melee and deckard is very squishy with little to no escape, his slow doesn't last long, and his root is easily dodged. Take Safety in Numbers if you're running low on mana, if you want more damage with your cc go for the Scroll of Stone Curse.