It all depends on how many of those horses you are using. Your boat will take the same amount of HP to propel it at a given speed no matter what motor is in it. The weight differences between motors is negligible. At best cruise, both engines will be putting out somewhere in the neighborhood of 50 HP on an 18' boat, burning about 4 gallons of gas per hour. The relative efficiency all depends on where each engine is at in it's power band. The bigger motor can easily be more efficient than the smaller one.
You may find this thread where Boating magazine tested the same hulls with different size motors interesting. At best cruise a Stingray 235 with a 4.3L got 3.92 MPG, the 5.0L got 4.42 MPG, and a 5.7L delivered 4.84 MPG.
A Grady-White 230 with a Yamaha F250 got 2.78 MPG, with a F300 it got 3.15 MPG.
http://forums.iboats.com/showthread.php?t=397741&highlight=interesting
Now if you feel you HAVE to use all those 190 HP in the 4.3L, it will DEFINITELY burn more gas at WOT than the 3.0L! In that case, it all depends on how heavy your throttle hand is ......