I can't answer your question with 100% confidence, but here's an 80% answer: use "tourism" as the purpose of your visit. I have a US passport, and once entered Japan for the purpose of transit, as I had an overnight flight connection to another country. So, I checked "transit" on the form (or, actually wrote it as it was just a fill-in-the-blank question at the time, and not multiple choice). After determining my situation, they changed it to "Tourism", as "transit" landing permission apparently requires documentation from the airlines involved, and it was just easier for them to admit me as a tourist. I would guess that the same would be true in your case, since you have a visa that allows for tourism admission. I think the only time they use Transit landing permission is in cases where the traveler cannot be admitted as a tourist and has a bona-fide transit situation, which they must document. That's my unofficial, 80% confident answer.