Advice regarding MCP48X1/49X1

Hello,

I am planning to buy Arduino Mega 2650. and then use a DAC to get an analog voltage at the output to drive an actuator.

My questions are: 1) If I use the PWM output which is 8 bit and then use a 12 bit-DAC , will it make any sense? or should I use a 8 bit DAC only. I do not understand the 8bit/12 bit working logic of the of the arduino outputs which will act as an input to the 12bit DAC.

2) The MPC49X1 has external Vref. This means my output voltage can go upto 10V if I give 5V as Vref. I am missing anything here. aslo, does external Vref has any disadvantage over internal Vref?

Thanks and Regards,