I always pick Chef for a gunman on heists because you can count on him not to die. I always hated it when I'd make the mistake of picking a crew member with less experience, only for them to get killed halfway through the job.
Why any sane player would choose any gunman besides Chef or Packie is beyond me.
For the jewel store I picked one of the less experienced gunmen, cause I figured that he'd become better after the job, while still taking the same cut. He crashed his bike during the getaway however, and I only avoided losing his part of the take because I'd picked it up without realising it. After that, I went for the competent people.