Twice and BTS are head and head when it comes to Japan in streams, album sales and stadium shows
Not really. BTS are doing 2x's more YouTube views weekly and their songs are doing better on LINE Music, Apple Music, RecMusic, and Spotify than TWICE ever since Dynamite was released. For album sales, TWICE is now struggling to surpass 100K sales on Oricon when they were able to do 300K-400K in 2017-2019. BTS, on the other hand, are doing 450k+ with ease with their Korean albums & 700K+ with their recent Japanese full album (first time for a Korean act since SNSD in 2011), their singles could probably do 1M+ now since 'Lights' sold ~800K in 2019 (before Dynamite).
Butter is basically #1 on all streaming and digitals sites in Japan (broke Spotify record of 500K+ while TWICE has yet to do 200K). Based on Soundcharts, TWICE have ~500K Japanese monthly listeners from Spotify and BTS have ~1.1M (will increase after Butter).
dynamite bigger than TT?
Yes. TT can't really compare to be honest.
Weeks in Top 10 on HOT 100:
TT - 13 weeks (only entered top 10 when Japanese version w. physical release was dropped)
Dynamite - 40 weeks (still charting)
Oricon/Billboard Streams:
TT - 80M+ in 4 years(Korean + Japanese versions)
Dynamite - 350M+ in 9 months
Oricon Downloads:
TT - 100K downloads (JP version surpassed 100K in 9 months but the Korean version was out for ~16 months)
Dynamite - 410K downloads (surpassed 100K in 3 months and 250K in 6 months)
* Dynamite charted in top 10 for Japanese radio for 20+ weeks and has been in top 40 for karaoke for nearly 9 months despite being an English song. English songs rarely enter high on Karaoke charts (dominated by Japanese songs).
* Dynamite has ~1.3M views from the two music videos (bside - 500K & original - 800K) on LINE Music. It is the most watched music video ever there. TWICE's most watched has around ~230K views).
* Dynamite is still in the top 4 most streamed songs weekly on Billboard after 9 months and top 3 on Apple Music, Youtube, and Spotify in Japan.
To see which band is most popular for the Japanese GP, you should check out the Oricon Karaoke chart. Whichever is on the list and whichever is ranked higher, it's more popular. Currently, neither group's songs are in the top 20.
English songs pretty much never enter the top 20 on Oricon karaoke. They even struggle to enter top 100 on the Billboard karaoke chart.
Basically, BTS is bigger than TWICE right now, both fandom and general public wise. It even seems BTS will have a bigger peak than them in Japan because Butter is looking like a hit by how it is charting.