Drontal is an ancient drug, so has years of data behind it. Milbemax is much less so. So newer drugs Stay on prescription for a few years, while sufficient mass data is gathered on it (or indeed no data is gathered at all, if there are no issues). All drugs start out with a veterinary licence, this costs millions to get, then over time better drugs replace them and they get down graded, and eventually can be brought over the shelf. Drontal is ok for round worm and some but not all tape worm, milbemax does round worn, tape worn, lung worm etc, it is a next gen wormer so less resistance.
if you don’t have an issue with the above then drontal is ok, but if you live in a lung worm hotspot milbemax is best. This is the advice of my wife (vet)