Practically speaking, the potty seats are anytime better than the chairs as they have to sit on actual seats and this helps ensure the little ones get used to the regular toilets and will be able to acclimate to toilets in general, not get intimidated by them when you're not with them. You wouldn't have to dump out anything and clean the dump several times in a day like you have to do with a potty chair.
But, if the toddlers are not reaching the regular seats even after you provide them with the clean and slip-free step stool, potty chairs must be used to avoid any accidents until they can use the toilets in general by 2-3 years of age.