What is the difference between those 2 logic gates symbols
The circle notation can be either at the input or output, and this is pretty widely used particularly in integrated circuit circuitry documentation.

This can be viewed as an example of assertion level logic which can help with a proper understanding of the circuit because in general this makes the circuit easier to read.

In this case, when CLOCK is low, the output of the NOR is low, and the input at C is high.