I want ot start martial arts and I don't know if I should do Brazilian jiu jitsu or isshinryu karate

I do taekwondo and I started shotokan karate a few years ago. Most martial arts school offer a trial period or at least one free class, I suggest you try each style and see what fits you best. I only know a little about the styles you mentioned from watching TV but they are quite different. You will probably get different benefits from each:)

Every martial arts are great. What do you want to learn it for, do you want to do events and go for trophies at national events ?

Brazilian jiu jutus is much more hands on, and physcial.

I would personal recommend taekwondo ( and perfer the Korean form) even amount things like taekwondo there are several types.