Senna is AD Lux 2.0 basically
Lux has a Shield Senna has a Heal Lux has a aoe root Senna has a aoe Root Lux gains bonus damage on her auto attacks vs targets affected by her spells Senna gains bonus damage on her auto attacks out right Lux has a 1,2 Ult 100-0 Combo Senna has a 1,2 Ult 100-0 Combo (you actually do the same combo different buttons) Lux has a huge range Ult Senna has a even more massive Range ult that also shields.
also note that at 180 stacks (very doable), Senna can take turrets outside turret range.