The first choice to make is between an AIS receiver and an AIS transmitter. With the first, you can see the transmitters around you. With the second, you can also be seen by the ships with a receiver, when they don't filter out class B transmitters.
I decided that I wanted to be seen but I haven't found a marine
VHF with an AIS transmitter capability, so I just ordered a separate transmitter.
The second choice is between "black box" devices and those including a display. If you have a chartplotter
, a black box is sufficient.