If you're running many apps simultaneously, like listening to music in iTunes and burning a CD at the same time, the dual CPU system would be more ideal because it'll distribute the work load across each CPU so you don't notice any hiccups in performance. And if there's one single app that's SMP aware it'll run a little faster as well. The G5 is architecturally faster but ultimately it depends on how it is used.
The official launch date is "end of August" but when the iMac2 (G4 + LCD) first came out there was a supply shortage so it may be a few months before the G5 systems really become available to more people.
