compare 2 engines to decide which is better?
 

I have two identical 1990 Cummins 12v engines and need to decide which one is a "keeper"... they both are in running trucks at the moment and run well, one auto and one manual, but they are about to be pulled in the next few days for a swap... what could I be looking for to decide which of the 2 motors is the better one??? either specific to Cummins or diesel motors in general...