The 75mhz is more popular, because they have more range to choice. Why you want more range? more option of frecuency is for to avoid interference or conflict. The 27mhz yo dont have a lot of option to choice.

"do you want tx, rx or both "
tx = transmition
rx = receiver
With this information, of course you need both.

"what is a module "
One of the big advantage of pro radio is they have the modular option. You can change a better modular for you request.
You also change the module for change the frecuency 27 or 75.
I can recomend to buy the spektrum module.