Tokyo, Kyoto, Osaka: which pack
The classic first trip is Tokyo, then the Tokaido Shinkansen to Kyoto, then Osaka next door, with a day trip to Nara or Hiroshima. Ten days of that, leaning on maps for every train change and sending photos in the evenings, sits comfortably in the 5 GB pack, which runs for 30 days.
The 3 GB pack lasts 15 days and suits a short stay in one city, or a trip where you are back at the hotel wifi every evening. If you watch video on the long train rides, share data with a travel companion, or stay two weeks or more, 10 GB is the calmer choice. The 20 GB pack is for a month in Japan or for working from a laptop through the hotspot. The data calculator works out a number for your own itinerary.