the pro for the pi is you can program it realy easy. you do not have to program the ardione. the i2c communication is more responsive compare to the usb usb-ttl uart interface (i know you could directly connect the serial port but usb is the simpler one)

if you use a arduino and programm the movecommand with buffers and so on, you can get a smoother movemat, but you have to code mutch. if you use an arduino you get a better execution time prediction compard to the pi solution.