Because parkour and freerunning share many movements, ideas, and philosophies, practitioners generally do both. Rarely will you see someone exclusively focus on one or the other. Once practitioners get immersed in parkour or freerunning, they usually take interest in many other kinds of movement based activities such as breakdancing, martial arts, dance, or gymnastics. Parkour and freerunning will teach you nearly all the same things and provide you with the same benefits. In the end, it is best to not worry about restricting your movement to one specific style or discipline. You can learn from all of them; the most important thing to do is just get out and move!
The basic difference of parkour and freerunning is that parkour focuses on teaching you to get through your environment and conquer mental and physical obstacles in an efficient way, whereas freerunning is about the freedom of movement. Being able to add stlye and flair where you happen to be. To have fun.