JBP different var

What is the difference between JBP (normal) and JBP kotobuki or other var.

Your question is at once simple and yet complex. There are many varieties of JBP. One of the better descriptions of this topic is contained in a text titled " Japanese Black Pine" by Steve Pilacik. ( 1993)
Chapter 10 is devoted to discussing the classifications of JBP. He suggests three groupings, Nishiki ( cork bark varieties) , Dwarf cultivars, and seeding varieties.
A true JBP produces white buds.
The Koto-buki is a dwarf variety. Characteristics include, white bud, blueish green needles, upright growth. It is one of the main dwarf varieties that has been under cultivation in Japan for many years.
The author discusses over 25 Nishiki varieties, 9 dwarf varieties , four major seedling varieties ( based on region in Japan) , and eight other variegated varieties with distinctive characteristics.
I am sure there is a lot of literature on this topic out there that others may mention.
Hope this bit of information helps to guide your search.

Thanks so much,
So which one is the most popular for Bonsai?

That depends on the individual. There are probably more regular JBP grown than any other variety, they make excellent Bonsai. However those wishing to produce smaller bonsai would likely prefer one of the dwarf varieties!
For a cork bark variety one of the popular ones is Kyokko. For a dwarf variety one of the most popular is Ban Sho Ho.
Keep in mind that local availability may be very different depending on where you live or what growers you can access. Also the dwarf varieties tend to develop much more slowly.

If the kotobuki is upright I’d avoid it - those types are usually for landscaping because they grow straight up and don’t take up much yard area, but it’s really hard to get the branches to go anywhere else but up in a lot of cases.