I often get asked, and wonder myself too, do I prefer commissioned works or making my own pieces to then sell. Many artists refuse to take commissions, feeling that it will compromise their artistic integrity. I can see their point, but if you work with your client right, this doesn't have to be the case.
Commissioned work is fine with me, as long as I can get along with the client. I certainly like being paid up front and having a solid job as I work. If the piece comes out different than originally thought up, I can chalk it up to experience and learning and start another one that will be the commissioned art piece.
If the piece is close to what was commissioned, and I feel it's a better work of art, I'll talk with the client, explain my views and usually the client agrees and accepts the piece. I don't think I've ever had a client that refuses and wants their money back. One was very close, but after a week, she called back and said she wanted it after all!
As for personal works, I do like to work on these, as they let my creativity go where it needs to go. Sometimes commissioned works can be this way, if the client says "Whatever you want to do!" (And I just LOVE those clients!!). I have so many interests and so many techniques available that I never run out of ideas for personal works. In fact, I have so many projects going, it is sometimes overwhelming! Sometimes, they sell and sometimes they don't. That's the rough part - not knowing if all the work I'm putting in will help me continue as an artist. I guess that's why some of the projects still sit, months and years after I started them, unfinished. I don't know if they'll sell, so I feel guilty about spending time working on them.
I wish I had the freedom to just paint or build what I want without having to worry about the business end of it, but I guess very few artists ( or anyone for that matter) has that kind of freedom, so I'm not alone.