healthy or healthful

Healthful has one meaning only: "giving or promoting health". Thus exercise, fresh air, good diet are all healthful.

Healthy can mean the same as healthful (you can speak of healthy exercise or a healthy diet), but it has other meanings as well. It can mean "having good health" (as in a healthy person) or "indicative or characteristic of good health" (as in a healthy appetite or a healthy complexion).

to the other question, healthy is more common than healthful.

Boephe

Healthful is an adjective regarding to whether an activity is good for your body or mind

Here's a quote from wordweb dictionary (free computer dictionary):
"Conducive to good health of body or mind", "a healthful climate"; "a healthful environment"; "healthful nutrition"; "healthful sleep"; "Dickens's relatively healthful exuberance"

Whereas healthy is more used as an adjective regarding people, or living objects.

As for which is more used, DEFINETELY healthy. I've used healthful maybe.... once a month? Whereas healthy maybe once a day. Also, the wordweb dictionary has way more definitions of healthy than healthful.

Hope this answers your question