First of all theI really hope this question is not intended to promote neither 2 extensions you mentioned, nor cheaper alternatives. If so it has to be shut down.
The fact that "everyone" is replacing OPC is a bit exaggerated.
The reason one may do it is the fact that simplified layout of the checkout page may possibly increase the conversion rate. However, this fact is questionable and might not be worthy spending $500+ (there are cheaper OSC extensions available but only 2 extensions you mentioned in your question are reliable).