Before we get into the details of what system will work for you, we need to explain the three different types of communication channels.
SIMPLEX COMMUNICATION
A simplex communication channel only sends information in one direction - from the sending device to the receiving device. Simplex is used only when the sending device does not require a response from the receiving device. For example, a radio station usually sends signals to the audience, but never receives signals from them; therefore, a radio station is a simplex channel. Another example is a microphone to a loudspeaker.
HALF-DUPLEX COMMUNICATION
In half-duplex mode, data can be transmitted in both directions, just not at the same time. At a certain point, it is a simplex channel whose transmission direction can be switched. Two-way radios are typical half-duplex devices. It has a push-to-talk (PTT) button, which can be used to turn on the transmitter, but turn off the receiver. Therefore, once you push the button, you cannot hear the person you are talking to but your partner can hear you.
FULL-DUPLEX COMMUNICATION
A full-duplex communication channel transmits data in both directions at the same time. It is constructed as a pair of simplex links that allows bi-directional simultaneous transmission. Take the telephone as an example. People at both ends of a call can speak and be heard by each other at the same time because there are two communication paths between them. Thus, using the full-duplex mode can greatly increase the efficiency of communication.
Ok, now that you understand what kind of communication you need to have, simplex, half-duplex or full-duplex, you're now ready to compare wireless systems based on this requirement.
DX100: Supports up to 4 full-duplex users or 15 half-duplex users
DX210: Supports up to 4 full-duplex users or 15 half-duplex users. Able to connect up to 4 base stations together for up to 16 full-duplex users or 44 half-duplex users.
DX300ES: Supports up to 3 full-duplex users or 15 half-duplex users. Able to connect up to 4 base stations together for up to 12 full-duplex or 60 half-duplex users.
DX410: Supports up to 4 full-duplex users in single-channel mode or 15 half-duplex users. Able to connect up to 4 base stations together for up to 16 full-duplex users or 44 half-duplex users.
FSII 1.9 GHz: Supports up to 5 full-duplex users per transceiver or a total of 25 full-duplex users with up to 10 transceivers. Able to connect base stations together for up to 65 full-duplex users when mixed with FSII 2.4 GHz transceivers on a system.
FSII 2.4 GHz: Supports up to 5 full-duplex users per transceiver or a total of 40 full-duplex users with up to 10 transceivers. Able to connect base stations together for up to 65 full-duplex users when mixed with FSII 1.9 GHz transceivers on a system.