Both are very good alternatives to conventional solutions of hanging curtains, however, both have their share of pros and cons. As far as curtain spring rods are concerned, they can be an absolute bliss in terms of installation, a process that can take no more than 10 seconds. You will also not have to drill any holes through the walls which can be a blessing.
Extendable curtain rods are in very high demand because of their flexibility in terms of area of installation be it shower stalls kitchen windows or any others for that matter. On the flipside, general models will not be able to hold heavy curtains for which you will have to look for conventional solutions.
On the other hand, curtain spring wires / stretch wires are literally unchallenged in terms of neatness and strength. The only thing that can come in the way though, is the installation process which can be a little demanding especially for those that are not experienced in the matter. Resource http://www.curtain-track.net/product/list-curtain-spring-wire-en.html