The 4.9 is harder on fuel than a 4.2 My family owned a 4.9L in an F-150 for 12 years. It was a very reliable motor, in fact the guy I sold it to says it is still running strong today. the 4.9 is capable of 300,000 to 400,000 miles. I now have a 4.2 and the fuel economy is much better. It doesnt have the low end stump pulling torque of a 4.9 but it is still a good motor and I havent had any complaints about it.

Overall it depends what you are trying to do with your truck. For some heavy towing I would want a 4.9 but for a commuter vehicle I would want a 4.2