Go by usage alone. For occasional research and email, choose ¥9.9/month for 60GB; for everyday use with a phone and computer online at the same time, choose ¥18/month for 250GB; for heavy use across several devices and frequent 4K streaming, choose ¥28/month for 500GB.
Route coverage, device limits and the refund promise are identical across the three; only the monthly data allowance differs. If you are unsure, estimate from your actual usage over the past month, or start on a lower tier for one cycle and upgrade later — the upgrade difference is prorated by remaining days, so nothing is wasted.