@gdavoll 

For myself, I make the network names different. The main reason is I know exactly what I am connecting to, and I don't want my device to move back and forth on bands to interrupt anything. Another reason is that if it is named the same as the main router I would not know if I am really connecting to the extender or the main router. It does mean I'll need to manually switch over, but can save headaches. 

Other users have made all the names the same and it worked fine because the device will dictate where to connect to.

So there is no right to wrong, mainly what you want.

I am assuming the Xfinity has a feature similar to our smart connect where it does that for you which is convenient.