Parallel motors is what we used 95% of the time. In the end it equates to more wattage.
Scale boats typically would use series wired motors more than we would as max power is not as much of a concern.
However, since the majority of us use BL motors, this no longer applies for motors/esc's.
EDIT:
In series, the RPMs will go down and voltage will be "split" between them the amps will remain the same
Current will drop since voltage drops. You will see less overal current for 2 motors in series vs 1 motor. In other words you will get more overal power from 1 motor than 2 motors wired in series.
Ryan