Antman vs Atom for me comes down to a couple different things,
both can shrink to sub-atomic sizes, but Ant-man i feel can shrink smaller via the microverse which has universes the size of atoms, and not to mention he can control not just ants but several different types of insects which gives him back up against Atom.
they're both geniuses but I think Hank is way smarter, even being labelled Scientist Supreme by Eternity, and has built some pretty insane tech.
Alongside this, both can grow huge but Ant-man can grow hilariously bigger as just as he can access the microverse by shrinking, he can literally become so big he becomes on the level of eternity and other abstract entities.