A craft makes something physical. A profession provides a service.
While opening a lock is a service, a locksmith also makes something physical for sale. I tend to think of craft as a higher priority than profession. In other words, ask if something is made first, if so it is a craft. If not then if it provides a service, then it is a profession.
Profession also allows a weekly paycheck. Synergies are where you find them, not limited to only Profession and Perform (BTW Perform allows for a weekly paycheck as well as Bardic abilities, so maybe there should be no synergies for Perform, either).
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Profession (bartender) <+2> Craft (brewing)
Craft (locksmithing) +2> Disable Device
Profession (miner) +2> Craft (weaponsmith), (armorsmith), (blacksmith)
Profession (hunter) <+2 Handle Animal
Profession (farmer) +2> Profession (herbalist)
Craft (alchemy) <+2 Spellcraft, Knowledge (arcana)
etc.
Ciao
Dave