What is the difference between first- and second-generation A-RGB?

Both generations use a 3-pin connector with 5 V, DATA, and GND. 
However, second-generation A-RGB offers more advanced data protocols, more precise and versatile lighting control, more complex effects, better synchronization, and smoother transitions. 
It can also support more LEDs per connector. 

In short: The second generation is an evolution with more features and better controllability.