I found city card very useful especially if you plan to see most of the paid attractions. This even includes the Untersberg Cable car, Hellbrun palace (Trick fountain) and discount on the Hallstat salt mine
This pass was well worth it. We got the 24hr pass and prioritized all activities we can enter for free because of the pass. We did other free actuvities on another day. This pass is enough for old and new town in one day. I dont think you need to get one for a longer period unless you really want to take your time or do activities outside Salzburg.
The Salzburg card gave us the opportunity to visit several local sites and some out of town places. A very easy bus ride out to the cable car to go up to the top of the mountain and see everything for miles. Many free sites and many with reduced fees. Well worth the money.