The lst 2 has a few minor upgrades from the lst, mainly the dsm radio (spektrum). If your son just wants to bash, the lst2 is a good choice. for racing applications, I would choose the RC18T. The 18T RTR will set you back about $145-165 and there are plenty of hop-ups available.
Both vehicles are good choices and both have plenty of hop-ups available. It just depends on what application you will be using it for.