I personally look at it like this. The japanese engines have unknown miles. With the american engines, more than likely the original owner will post the original mile on the engine and if you ask you can see pics of the odo to be sure. The american engine has the torque down low and holds power all the way up. The japanese engines that I've drove/ been in were all hp up top. Some may say that the japs get better gas mileage but how true is that if you have to rev it higher to get to the same speed as the american? I've read corvettes with ls1's getting 25-28mpg easy driving. Maybe the japanese engine can do that I'm not sure.
I would personally go with the ls1. I like american power and japanese styling. Unfortunatly I have limited funds so I'm going with a lt1 and if money permits later, I'll sell the lt1 and opt for an ls1. I think I'm going to try to keep my z32lt1 under 2 grand doing all the work myself.