Buy it when you get there. Why lug it through the airport if you can simply pick it up when you arrive?
This works great for disposable items and toiletries. But if your trip is going to be short you might want to pack these items instead so you can spend your time exploring instead of shopping for toothpaste.
Another thing to consider is how expensive the items will be at your new location. I haven’t found any concrete pricing on diapers (nappies) for Portugal but have read that they are expensive. The bright side to hauling a suitcase full of disposables is that you’ve got room for souvenirs on the way back!