Ideally you want both, with a slight positive pressure maintained. Depending on the size of the engine compartment and how easily fresh air can get in, too much exhaust
blowing can lead to inefficiencies as the engine (which is really also an air pump) is slightly starved of combustion air. You don't want negative (in relation to the atmosphere) pressure in the engine compartment.
On a small sailboat aux engine, this may have zero practicalities but it is very relevant on larger power-driven vessels with large displacement
If you have room and means to do it, have one blower drawing fresh air in and another blower as far across the space as practical, exhausting air out.