Well first of all I cannot even think of going to Japan just one time. I am the type of person if I like it I will make it happen again and again. The first time I went I went during fall harvest I can say I was overwhelmed with education and the fine art of selection and pulling the trigger to spend some cash.
I think if I went to the All Japan that early I would not be able to appreciate it fully. I think being at the fall harvest I got to see first hand how it all works in koi mecca. I got to see some of the best and some standard koi. It was good to see a comparison of different koi in breeders ponds. So I would have to say fall harvest. If you are seasoned with all that info back then you would be able to appreciate the All Japan show a bit more. JMHO...
I also have been in the spring for tosai and hand selecting tosai is a rush for sure.
Just like Ray, Jim and BobW I have been to Japan on just sightseeing as well. I don't ever think you could fit everything in 1 trip. I always try to but it never works out.
By now you will see why I have been back many times as a hobbyists and tourist. I can't get enough out of 1 trip
Joe