Lighting is based on plants..If you want High demand plants, get a HO. If you want Low to Medium lighting go with NO.
HO (high output)
NO (normal output)
Lighting should ALWAYS be the last upgrade, however most of the times it is the first..THEN you have over bought and now saving up for Co2 because you have to much lighting.
SO the questions is WHAT do you want to plant? Do you want to go LowTech / HighTech?