C is good for this microcontrollers that we have today - a lot of flash, couple of hardware timers etc.
You use ASM if you really want to squezze most of you microcontroller or if you do some time criticall stuff... and it is sometimes good for debugging. But ASM is different for different MCU, whereas C is basicly the same everywhere. And sometimes you will swear about simple stuff like comparing two values and doing something based on that comparison in ASM... which is a piece of cake in C.
So my advice is to learn C and spend other time on learining about electronics, motors, sensors, etc.