They're both first generation antihistamines and work as agonists on the H1 receptor, causing sedation. There's no reason to choose one over the other for a first time user, but they can cause rapid tolerance. So, I'm guessing the only reason they offer both is if you'd become tolerant to one of them and can no longer fall asleep on it.
pogue|3 months ago
https://en.wikipedia.org/wiki/H1_antagonist#First-generation...