Flying into Vienna for Bratislava
Many visitors to Bratislava never use its airport. They fly into Vienna, which lies between the two capitals, and take a bus from arrivals that reaches Bratislava in about an hour. That affects which pack you buy. A Slovakia pack does not work in Austria, so on that first stretch it simply waits, and its 15 or 30 days only begin when it connects on the Slovak side.
If Vienna is just where you land, the Slovakia pack is all you need. If you are also spending time in Vienna, or carrying on to Budapest, the Europe pack is the better buy: Austria, Slovakia and Hungary are all among its 33 countries, and the eSIM keeps working as you cross. Either way, install it at home from the QR code in your email before you fly.