They are totally different animals with both their own features and drawbacks. They are pretty drastic differences so like he said what do you want, because neither is "better" than the other, it's just a matter of what you want out of the truck. I think V-8 fans would reccomend a V-8 and inline fans would reccomend an inline
You should get (AVERAGE) just under 20MPH with a 250 and a 3 speed tranny, and just under 15 with a v-8 although this could be discussed and disputed forever, with peoples individual experiences hehe A 292 is going to be more inline with a 350 on gas mileage.
You should take the swap itself into consideration too.... radiator, shroud, tranny lines motor mount perches, crossmember, and driveshaft might all need to be moved/cut/extended.
The integral head 250 is fine if are ok with stock, and have no ideas to upgrade, otherwise you are looking at swapping to an older style head if you want to add headers and an intake to the inline.