what is better for a street fight, Japanese karate, or American kempo?

The one with a good teacher. Martial Arts work if you're taught them right. Style makes no difference. Kempo and Karate have different styles under their names. Too many to cover here.

As mentioned no style is better than the next but if you go to a good school and have a great teacher thats what matters.. Determination and effort as well make it

But based on those styles you ask about it's basically preference and interest as I take Ed Parker American kenpo and find it effective and interesting. It teaches through natural movements and powerful quick strikes but many styles of martial arts have similarities and differences

neither
i say become a bare knuckle boxer

then youd kiss ass and take names