I worked on both but now I work only on paid per minute sites. I know there possibly is better income potential in free chat as more guys visit those sites but most of them are freeloaders. I must admit that my highest earning day was on MFC but my earnings from PPM sites are more constant and at least I'm sure I will pay my bills and taxes...
I loved Chaturbate but now I cannot get it to work for me and I got capped and posted on pirate sites. But I know girls who do better on free chat based sites.
What's working for me, might not work for you and vice versa