Two differences how they generate the code and how they send the code, The kind of code is the same. (Gcode)
Niether save the file to the smoothies sd card both stream over usb.
You can save the file on the smoothie and run it from there, this will prevent any interruptions due to the computer disconnecting or somthing similar.
I cant help with MAC issues as i dont have one, was it happening before? What changed?
You can try using a better/shielded/shorter USB cable, Try chaning the port you us on the mac,