A 300 stomps the 4.2 as a truck motor, and I've had both engines. I had a 4.2 in a 99 f150, and have a 300 in a 88. the 4.2 has more hp, but will have to rev to get any torque. the 300 makes it at 2000 rpms. it was in f250's and 350's. Dt and 390gashog know what im talkin about! Hell, My buddy has a '99 f150 with a 4.6 and my 300 has more torque! they are both rated at 265 ft.lbs, but i got it at 2000, not at 4000 or what not. as for reliability, the 300 is a 300k+ motor when maintained. I have no idea about the V6. but the V6 might get more mpg's. It depends, i dont think there are 2 300 I6's ive ever seen with the same mpg's.

performence wise the 4.2 is quicker, but i doubt you can move around easily with a nice heavy load makeing the bed sit low like you can with the I6.