I am a fan of the 4.6is due to its rarity. The real concern in my opinion to watch out for is the timing chain guide issue. Once you get to 100K just run through it on the maintenance and do the cooling system. The 4.6is has rear air suspension and normally does not cause a lot of problems like the 4.8is all around air suspension. Fuel economy will not be that much greater going from a 5 speed to the 6 speed in these trucks. Not even 1 mpg of a difference.