of course get yourself MTX4.
MTX3 is an old car by now...... and you will find it VERY difficult to find spare parts like bulkheads, suspension arms, upper deck, and many other things. And you will be alone driving an old car in the track.
Secondly, MTX3's handling is no contest to MTX4 ( I have mtx3 and mtx4, and I know the difference )