I used relays cause I didn't want to wait on a box - and - the relays were selected to be 10mA of draw 

 - easy as, minimal drain. 
The transistors would add load to the circuit just around 1/10th of the relay - all insignificant though.. 
I got the relay box together the same day (saturday). 
Why the relay box or the other box? Simple - they don't want the globes of the trailer (500mA + ) of load on the existing circuit - hence why the relay box is just as valid as the transistors.