It all depends on your personal preference, your needs and so on.
Just because it's ModernBill is better for one person does not mean it will be better for you. Just because ClientExec is better for another does not mean it is for you. Just because WHMAP is better for someone else, doesn't mean it is best for you.
Don't choose something because it has the most features, because it is cheaper, etc.
Always compare the two and look to see which one does what you need done easier and better. There are always differences between two products, and one may be better for you than the other while it may be the other way around for another person - one of the reasons I don't like threads asking "Which is better? A, B or C?" .... because for me or the person around the corner it could be A, while for you it could be B and for the rest of the world it could be C.
There is no best.... there is only the best for you.