I'd like to order a lighting unit online for a 48" hood but I don't know whether to go for a 42" one or a 48" one? Would a 48" one fit?

Which size would you go for if they would both fit?

Cheers

I'd go for the 48" one. That way you get lighting the entire length of the
tank. With the 42" you have dark spots on each end of the tank. Looks
kinda gloomy in a way.