Bigbang edges BTS in Korea when it comes to the general public. But BTS have the whole world including Bigbang's touring turf/second home aka J-A-P-A-N.
Nonetheless, if we look at a bigger lense- Bigbang is not even remotely a competition to BTS. This is like asking a comparison between IU (mostly popular in Korea like BB) and BTS which is WhyKnock specialty 😂
Well now you are expanding the scope.
Then it can be asked whether
IU or Bigbang, who was more popular in their peak
AND
IU or BTS, who was more popular in their peak.
When it comes to S.Korea, the question still can be debated.
Possibly in 2032, we can add,
IU or XYZ, who was more popular in their peak.