I don't think it's a good idea to include these, as they do confuse people a bit, and can also be conflated with the total system capacitance which would lead to poor results.
The small capacitor that yields most of the input capacitance of 120pF is 100pF, and has to be connected after the RF stop resistor of 100 ohms or so to prevent radio-frequency interference in the VHF band from getting onto the input where it would be envelope-detected as audible hash. If it were to be routed through a switch, the extra inductance would make it a less effective shunt.
A lot of the preamplifiers advertised as 100pF are in fact 120pF or more, as you've got to factor in the PCB track-to-ground capacitance, op-amp input, and so on...
Adding more capacitance can be done quite easily, but adds complexity to the design, and only really affects the response above 10kHz by a few dB at most. Most modern cartridges work best with about 200-300pF, so I don't really see a need for it if we have 120pF from the preamp and then another 80-150pF from the tone-arm/cabling. Adding it would just increase the size/complexity of the board, enclosure, generate long e-mail exchanges about setting it, give people the opportunity of setting too much of it and complaining etc.
If it's not necessary, then it's best avoided. Keep the phono-stage simple so it can be plugged in and work straight away with the vast majority of devices, with the loading question completely out of mind.
The same goes for MC loading, which doesn't even really affect the frequency response until it drops below the coil resistance or less. You need a low enough resistance value that lets the resistor dissipate the resonance of the cartridge inductor against the load capacitance. The load capacitor selected for its highest value such that the appropriate resistance is about ten times the coil resistance to keep insertion loss under 1dB. If you design the input amplifier correctly, then the thermal noise of coil resistance will dominate if it increases beyond 30 ohms, such that the noise penalty from insertion loss is mitigated for higher coil resistance.
If you let the user select the MC load resistance value, then they'll either increase insertion loss and harm the signal-to-noise ratio of the device, or create an underdamped RF peak at the coil-inductance/load-capacitance resonance point that will be detected on the input and lead to complaints. Changing the loading alters the level at the input slightly, but perhaps enough for people to convince themselves they have a 'preferred' amount that will probably provoke trouble... Most preamps on the market use pretty brutal (and rather noisy) resistive series RF stopper networks on the input amplifier, preventing the resonance from getting through, at a massive noise penalty which is only acceptable because the market has such low expectations with the intuitive (but incorrect) explanation that the level is lower and therefore noise is correspondingly higher.
While selectable loading is very easy to implement, it is not necessary in my opinion and actually highly undesirable from a customer-facing point of view.
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