There are people that advice to buy new car because you have to build it on your own and teach yourself how it works. I think it's quite obvious if you buy used car that you'll disasseble and build again just to check if everything is ok (at least I did it with every used car I bought - no matter who was selling it to me). Also experience from our club is that some people bought Tamiya's here and despite lots of tuning (in the end probably more expensive than top car) it didn't perform as they wanted. Then they bought used top cars and they improved their times immediately. They didn't have problems as before anymore. That's just my experience and opinion.
And yes you probably won't feel difference between 2 newest models, but you'll feel difference between new budget car and used top car.