Sorry ill beg to differ here .......... Yes the Kokusai , is based on a real race car so it has more pedigree/history, however i think the fact that the Stealth edition is a "blessed by Polyphony vehicle" it is the better choice.
It looks better, has a higher PP rating stock than the Kokusai and is a good deal cheaper, will deteriorate slower than the Kokusai and is cheaper to maintain in the long run. It also has a rear view modelled whereas the Kokusai doesnt being a race car.
I own both and have done considerable mileage in both. They handle very similar as can be expected as it is the same vehicle, however fully modded or stock the stealth Mclaren seems to have the edge. It is virtually impossible to loose control of this car (with racing hard or above tyres) such is its pin point precision handling. It is also ridiculously fast in a straight line . Overall this is one of my "Go to" cars I turn to it when in need of decimating the AI or online Veyron bandits Ill sum up with a quote from Samuel L Jackson "When you absolutely positively have to kill every mother****** in the room" ........Turn to the Stealth Macca !!!!
If you must get one get the Stealth.