The syntax is quite similar. And you can actually code pure C in the Arduino IDE too, but it is not meant to use it this way.
Arduino is based on using librarys for everything. Most of those librarys are quite complex and therefore huge. For many projects you need only a few features of what the lib offers. But it all compiles. Arduino IDE also includes needed libs by itself. That can be a pain in the ass ^^
I use AVR Studio and managed to compile the same functionality i needed to 1/8 of the Arduino size. Arduino is for people that want a quick setup or don't know much about the microcontrollers. For a release product it is not a good choice. If i just need a pwm signal or anything like that in no time i like Arduino
