Polyphony is the number of notes that can sound at once. In practical terms, it's a measure of how complex a piece you can play before it becomes noticeable that some notes are being cut off prematurely. For example, on a board that has 4-note polyphony, it is very noticeable in most piano pieces.
Sixteen is the practical limit for piano, unless you're using the sustain pedal a lot or playing very complicated pieces, so either of those boards would do fine. 48 is of course better than 36, but you probably wouldn't ever notice the difference. However, on pro-level digital pianos, when using the sustain or sostenuto pedals, 64 or 128 is desirable because an acoustic piano can generate sounds with all 88 notes, even if you're not playing all 88 keys. These high-end boards simulate this effect and can do it more completely with more notes of polyphony.