What do you mean 'setup additional networks'?
I've not used a range extender but it's my understanding that you plug an ethernet cord into them that's connected to the network in some way and then it creates wifi where the signal couldn't reach before.
And I've also not used a repeater, but it's my understanding that you set them up right on the edges of your wifi network, it picks up the signal, amplifies it, and sends it back out.
Neither method, in my mind, requires an additional network. Plus the goal is to connect to your existing network anyway so that would completely defeat the purpose of using the device.
The second device would be nice because it won't require you to run a network cable to the location where your dead spot is. Also, fwiw, the extender has some pretty negative reviews on Amazon. It only has a total of 3 stars (which is relatively low for amazon).