A 108,000 mile engine has almost 50% less miles than a 155,000 mile engine, and for a cost difference of only $600, I cannot see why taking the higher-mileage engine would make sense. Also, since the original engine did not make it to 200,000 miles, it is highly probable that the useful life of this type of engine is less than that.
If we assume, for the sake of argument, that the useful life of this type of engine is 160,000 miles, the 108,000 mile engine still has 52,000 miles left in it; the 155,000 mile engine has only 5,000 miles left in it. For a paltry $600, this truck could be driven another 3-4 years at the current rate of mileage accumulation. That’s a real bargain if you ask me!