AE to extend a TC network question

I have a wireless network established with my Time Capsule and I have an Airport Express connected (through Ethernet) to the network and then have it wirelessly extend the network.

When I go to connect the AE it asks me to select the Device or Network I want to join and extend. I have two logical choices, my established network or my Time Capsule. Both show up in the list.

I chose my network and everything seems to be working fine but would it make more sense to select the TC? Does it matter or will it just do the same thing.

Thanks for the help.

 

Any thoughts on this?