I would go by what you want the skill to do. Realistically, I like to define Profession skills in terms of making money. Craft skills is used in terms of making the actual object.
A couple examples are helpful here. Let's talk sailors in the first one. If a person is walking along a slippery deck of a ship, I'm not going to have them makea profession (sailor) check. I'm going to have them make a balance check. Even if they have 80 ranks in Profession (sailor) theycan still slip. Profession (sailor) just tells how much money they can make at a job over a given amount of time. Profession (sailor) says that they knowhow to do job, but it doesn't actually find use in the nuts-and-bolts of doing the job.
As a second example, let's talk actual painting. If someone is wanting their character to work for a month as a painter, that's where I would use Profession (painting). They'd be able to figure out their daily/ weekly/ monthly/ yearly income if they want. But if someone needs to create a specific painting for someone that they are trying to impress (or bribe, or whatever) then I would use a craft (painting) check.
I'm not claiming to be right here, but this isjust how it makes sense in my mind as I type.
