I'm looking for trade-offs which I should consider when choosing this
frequencies but I can't seem to find anything really specific (in
theoretical or equation form), other than the bandwidth.
There are no trade-offs that are general in nature. The bandwidth of 3 kHz to 95 kHz for any one particular installation will, very specifically, have some spectral areas that are more suitable than other regions. Then, for instance, down the street, someone else using that bandwidth will find that their "optimum" points in the available spectrum will be totally different to yours.
In other words, a particular PLC system will find areas of the spectrum that are suitable for its transmission and avoid areas of the spectrum that are not suitable. What makes one part of the spectrum suitable and another unsuitable is totally dictated by the specific loads, supplies and wiring regimes in that particular application and not by any generalizations.
Then, on another day, the PLC system might change to use different parts of the spectrum to suit the prevailing interference on that day. It might even recalculate the hot-spots to avoid at an almost minute-by-minute timescale.
Do you guys know of any other considerations when choosing these
frequencies, other than that the demodulator should be able to
demodulate the signal, such as signal to noise ratio, bit error
probability, etc?
If the transmitter (the master device) sends out a specific message using two particular FSK frequencies and receives a report back from the recipient (the slave device) then it can be assumed that those frequencies are somewhat usable in the short term. This has to be repeated as often as is necessary to ensure that those two frequencies remain fairly usable.
A decent PLC system will continue (in the background) to "test" other frequencies that might be relied upon as a fall-back. It's not beyond the realms of possibility that both master and slave will passively examine all parts of the available spectrum to check for interference and transfer those "findings" between each other so that there is a fall-back position should comms be suddenly lost.
