Hello,
Well-written rhinocommon code will almost always be faster computationally. Whether that difference is significant for your use case is another matter and depends very much on what functions you are using on how many of what type of objects. The ease of writing, debugging and updating rhinoscriptsyntax may be much more valuable to you than computation time.
Rhinoscriptsyntax is all written in rhinocommon and if you are chaining multiple operations together then each rhinoscriptsyntax function may repeat some operations, getting a guid identifier for the object, doing some checks, carrying out the operation and then applying the identifier to the new or modified object where in rhinocommon you could just do the operations you want, possibly carrying out the checks once rather than multiple times.
This tool will help you find the underlying source code for a rhinoscriptsyntax function 
-Graham