You can get them both for less than 20 dollars at any hardware store, no reason for it to be an either/or.
So you're making a trip to the hardware store, and standing in front of your door with a power drill you dug out, found bits for, etc. ...
... And only one of those devices is getting installed?
I'm not saying you can only have one, I'm saying that they both preform the same function. They both allow you to see who is at your door without having to unlock it completely. Why have both?
If you have a peephole when would you use the door chain?