Not having even looked at the engine (C4), ask yourself these questions...

1. What is my goal?
2. What features are crucial to achieve that goal?
3. What degree of flexibility do I have?
4. What is my budget?

You should be able to answer those questions without even going online, much less looking at engines. After you have answered those questions, start comparing engines (GSPro and C4 in your case). Other peoples opinions on this matter will do you little good, because they do not directly pertain to what you are trying to do.

Open up both engines feature pages and pick the one that best fulfills those questions.

Regards,
Raddaddy