They're both short for John. Jack is English, Jock is (traditionally) Scottish.
In England we sometimes refer to all Scottish people as 'Jocks' in the same way Americans use 'John' (as in 'John Doe'). In this way it could be cliched.
It can also be seen as a pejorative slur, depending how it's used, particularly by the English against a Scottish person.
None of its to do with how Americans use the word 'Jock', which comes from 'Jockey Strap', used in sports.
No one in England or Scotland would call someone a jock if they were into sports, over here it only means Scottish!
None of that probably helps your decision