I use Sim2Fly. It's the best ever! Fast and cheap. All the Japanese local SIMs for tourists are very slow, and of course more expensive. I had been using tourist SIMs since ~2010.

SIM: cheaper than pocket wifi. Can be tricky to set up for iPhone. Some people simply couldn't get it to work. I never had a problem using models from 3GS to iphone X.

Pocket Wifi: unlimited (subject to fair usage), very high speed data. Most people do not need that kind of speed. Extra equipment to carry - the device itself and external battery pack. Everyone in the party must travel together to benefit from it. Coverage is not as good as SIM (which is NTT Docomo for Japanese SIMs, the best coverage). Sim2Fly is on Softbank. Some cheap pocket wifi typically provided by Airbnb hosts have dismal coverage in popular out-of-town destinations, or underground malls.