I have seen both here in Spain, and the criteria is unclear, as every operator is different, and even an operator has a different criteria depending on the site.
They seem to agree in an antenna migration to be able to allocate new frequencies. Most of the 3G is being migrated to mixed mode 3g/4g.
What we see here mostly is NSA. IOT cells I have seen so far are B20, defined on some LTE basebands.
A matter of opinions really. You can keep height of 5G antennas similar to 4G antennas, but if you can’t and you need to stack them then I prefer to put 5G antenna above a 4G passive antenna, for following reasons:
Our 3.5GHz spectrum allocation has much larger bandwidth than a remaining 4G, thus the biggest throughput contributor is at “prime location”.
3GHz band has worse link budget, thus again should be higher to risk less obstructions than lower bands.
3GHz band has worse diffraction, ie waves “propagate less” behind obstacles, diffraction angle matters, at least in theory.
If you are using MaMIMO, then such antenna typically has much larger “scanning” range with your beams, thus it’s better to place it higher to avoid rooftop clipping…in a case you are not at the roof edge, but somewhere at a center.
Latest MaMIMO antennas are smaller and lighter than multiband 4G antennas, thus if you need to put one antenna higher… it’s easier to choose 5G.