Simple answer.
Mech Mod; Cheaper, not as safe as regulated mod. Can be more reliable since there are no electronics. Have to use high quality
batteries.
Regulated Mod; Higher price tag then Mech mod, way safer. Can be less reliable to do complicated electronics, but many last for a very long time with no issues. Dont have to use high end batteries if you stay with in batteries range.
Sub-ohm; bigger clouds the lower the ohm, more flavor, uses less Voltage and Wattage to achieve the goal.