Usually MHz mainly just decides what signal it responds to (useful if you want to have more than one running around at the same time).
There are considerations once you get to higher levels but frankly distance, interference and penetration concerns are not really worth the effort of thinking about at this stage not to mention once you do you get to that level your car will be able to swap out crystals at will (or indeed just press a switch).
Long story short- pick whatever one you like the shell, features and power of.