...no wait... i guess i do have some things to add to this thread:
Yes, although gen~ code is based on the language of 'LUA', the basic conditionals and other such syntax of C which are used in many languages like Java, C++, etc. are also in gen~ so it can be pretty easily adaptable(LUA is based on C same as many of these other languages).
The fundamental differences in performance are minimal(the DSP routines themselves work pretty much the same calculating things at sample rate and allowing you to hide functions from that calculation within conditional statements wherever necessary). There are some advantages to writing an external in C which have to do with having direct access to the internal 'struct'(or memory of an external) and performing efficient checks as to whether a signal or a message is attached to the inlets of an external, then catering the DSP method to those(whereas in gen~ it's all assumed as signals by the time it hits the DSP method within, but certain types of messages are only calculated once per vector....), however, most beginners won't find these differences to be huge gains or losses.
The main advantages to writing externals in C are found in creating access to some of Max's more esoteric and special features and functionalities(as mentioned before, you can separate a DSP routine that works specifically for messages as opposed to signals(detecting what's connected to each inlet then moving from there), but other examples of extending behavior using a C external are: gaining access to specialized functionality within poly~, doing specialized checks and reformatting for changes in external buffer~s, creating behaviors specific to Windows as opposed to MacOSX within the same code, doing special things like handling files, creating a UI within the same audio-handling object, creating different typed outlets(for example in karma~ there's a regular set of signal outlets, but then an internal scheduler-clock polls certain information to be sent out the last outlet in messaged format), etc.).
Having said all that, if you're just starting with coding, gen~ is best since it offers a bridge between visual patching and coding(with codebox), and is still highly efficient, but eventually i've actually found writing externals in C to be a bit easier simply because i know the SDK and it allows me more modular access to different kinds of functionality and efficiency within one external/code(easier in the sense that i can separate things out to be more readable and develop them in a more organized easy-to-keep-track-of sort of way).
Anyways, best of luck and have fun!