They all have equal chance, but clearly when you fail you have 0% chance. So with that in mind I'd advise you to do the first job.
The drop rate for Extra Credits is a 1% chance every time you successfully Complete a job. Now this doesn't mean if you do the job 100 times, you are guaranteed to get one, it just means you have a 1% chance each time you successfully complete a job (0% chance if you fail). For Doctor's Notes, the percentage seems to be less than a 1% chance every time you successfully complete a job (0% chance if you fail a job).