Why? Margarine contains a big amount of trans fats (15%) that are universally known in the scientific literature as dangerous for the health at any dosage. Butter contains much less trans fats (3%)
maybe some can disgest animal fats not so well but i have no proof for my claim so i guess your argument pointing out the trans fats is way more legit.
and i know that trans fat is dangerous or at least makes some dangerous sicknesses more likely