Answer: I prefer doing suburi with a bokuto (or bokken). It is simply because it gives me a sense of a sword. In addition, it is a good idea for beginners to use a bokuto too. The reason is the same as above; a sense of a sword.
But in kendo we use a shinai. So it is important to do suburi with a shinai too. Shinai is longer so the balance is different. You have to get used to the shinai you use; otherwise, you feel awkward when you do kendo with that shinai.
Do a suburi with both. In that way, you can still feel the sense of the sword and also get used to the shinai you use.
Do you think the late starters become 8-dan? I mean those who started kendo in their 30s or 40s. My father started kendo when he was 55. He passed away 1 year or even less before he was going for 6-da…