A work for hire is when you sell your image outright with no restrictions or expectations about any further monies or royalties or license fees ever in the future. It is a REAL ... final sale of a product, ... like if you went out and bought a can of Spam - the Spam is now yours and yours alone forever to do what you will with it. You OWN the spam.
When you take on a work for hire, you sell ALL your expectations of any future rights to monetary gain from it. The sale of it is your ONLY monetary gain EVER.
A commission COULD be a work for hire, if your contract/agreement stated this. But it does not have to be. A commission merely means that you are hired to create an image (painting) for a particular person for a particular use by that person, and you retain ALL copyrights, ... meaning that you can paint/photo the image again, sell it again in any form, license it in any form at your will at any time simultaneously or in the future.
In one case you are getting paid for your intellectual property right as well as your image. In the other case, you are getting paid for only the image in its contracted use.
At least, that's how I understand it.
I tend to think that someone who wanted a commission would NOT want the image in a position to be duplicated. You just have to be clear about what you are doing in your own mind, and be willing to make this clear to whomever wants the commission. If they want all rights, and you are willing to work this way, then you basically are selling them the painting totally, forever. Chances are, the person who wants the commission sees it as a personal piece, and the idea of having it in a position to be mass produced would detract from this and discourage the whole deal.
The one commission that I have done [Okay, laugh], I would NOT put it in a position to be mass produced, unless I checked with the person for whom I did it, to see how he might feel about it. I don't think that I would put that image here for anybody to buy. I might consider certain licensing situations, like a book cover or magazine cover, but I would ask the person for whom I did the commission first, and probably I would have a credit note in the publication saying the name of the person whose art collection it was in (if the person felt comfortable with this).