Is a universal all-in-one adapter or a country-specific adapter better?

It depends on your itinerary. A universal adapter covers US, EU, UK, and AU sockets in one unit and is ideal for multi-country trips. Country-specific two-packs (such as the EU-only and UK-only TESSAN sets here) tend to be more compact, sit flusher against the wall, and give you a spare, which suits travelers who repeatedly visit one region.