A Japanese would be spotted more easily. A foreigner "interested in Islam" would blend it better and could penetrate these supposed 'cells' in Japan and help shut them down quickly, or a least cause suspicions and make it more difficult for them to gather.
I have always thought Japan should use 'good foreigners' in Japan to uncover, combat and expel 'bad foreigners'.
As some old timer 'good foreigners' in Japan read this forum, I am sure they are just as disgusted with me about how recently arrived 'bad foreigners' have been ruining the place and helping cast doubt by Japanese on all foreigners.