I'm not a techno geek,but I'll give all out from understanding.
AM can tranmit a further distance than FM,but its more prone to glitches or noise interference.And its cheaper than FM.
FM is a more popular choice of racers.As it is less prone to glitches and the area we run our cars is more then enough for the range.But its more expensive.
Maybe someone with more technical know how can add on for me.Thanks.
But if you're getting a radio,I recommand FM radios,if you're going to race in the near future(more advance radios only comes in FM,unless there's a AM convertion for it)