Selection of Frequency in Switching Regulator
There are several aspects to consider:


If you increase the frequency, the time between two cycles goes down, so does the energy per cycle. This means you can use smaller capacitors and inductors when you target the same ripple (and really low ripple is important to make use of 24 bit ADC resolution).
If you increase the frequency, you increase capacitive coupling (because the impedance of the parasitic capacitance between traces on the PCB decreases with frequency). So possibly you need better shielding or guard traces to avoid interference.
You need to avoid beat frequencies between the sample rate and the converter, so every sample gets the same disturbance (hopefully this results in merely a DC offset). One way to eliminate beating is to make the converter run at an integer multiple of the sample frequency (this means: you would not use the free-running oscillator at 1.2 or 2.4 MHz, but generate your own frequency by multiplying the sample clock).