Either way is fine. Use whatever method you can remember easier. It goes down to personal preference. I personally like
setMotorSpeeds(portx, z);
, but i know that my friend who teaches RobotC to all our new members teaches them to use
motor[portx] = z;
(I am using x and z just to say to any new team that any valid port or motor value will work.)
NightsRosario is right. I do lots of programming in RobotC and it is completely personal preference. I prefer motor[port] = x; a lot better because that is what I am used to, but it is your choice.
Hell, within a short period of time you will probably be using functions of your own for autonomous or control of your motors (with magical things like PID controllers and sensor integration). Personally I dislike natural language, because I dislike not having an intrinsic understanding of what my program is doing (even if I can view the function declarations), but it completely comes down to what you have been taught to use and what you want to use.
Happy Programming.