Thanks for the question, yes, I can see why this maybe confusing initially to a beginner in remote sensing. The simple answer is that there is not a right or wrong answer in regards to this. Both Band 8 and Band 8A measure similar wavelengths, as shown in the table below. You will get very similar results, irrespective of which band you use as both are measuring in the NIR part of the spectrum.
NIR, 832.8 nm (S2A), 833.0 nm (S2B)
Narrow NIR, 864.7 nm (S2A), 864.0 nm (S2B)
If you’re measuring your small areas then I would use B08 as its spatial resolution is 10m but it is personal preference. I would recommend reading deeper into the literature about this; also outside our documentation to fully understand the significance of different sensors, bands and wavelengths.