If they debut in Japan there is a large chance I-fans won't be able to see any Japanese music show stages or interviews. However, the j-line is more popular in Japan than Korea,(from what I've seen) and by debuting there, they would be gaining more exposure in their home country which is the largest Asian music market and the second largest music market in the world.
On the contrary, if the debut in Korea they will get more international exposure and I-fans will have an easier time seeing the girls perform and buying their albums. However, they will be an all Japanese subunit promoting in South Korea, which leaves them vulnerable to xenophobic comments by Koreans. They also have a higher chance of not charting well if the Korean GP decides it doesn't want to listen to a song by a all Japanese subunit.
There are pros and cons to each. I would personally love it if they promoted in both Japan and Korea, but I know that's not likely. If I had to chose Japan seems like the better option. The girls would be promoting in their home country, the music market is larger, and they have a better chance of charting well in Japan then in Korea in my opinion.