From my experience, they will tend to pick random work to do if they have experience in it (and they will also tend to just not work sometimes c:), although depending on the villager, it seems they may prefer one over the other if given a choice. How much experience they have in each area may also play a factor (i.e. they will do the job they have more experience in more often), though I'm not positive. If there's something that needs to be done, like if they're low on food or someone is sick, they will probably do that if they have the experience.