SRVCC: It is a single voice (VoIP) call involving IMS that is continued in different RAT. So I think the emphasis is on "single voice". Or someone in 3GPP was looking for a nice and sticky new acronym.
CSFB: there is no voice (user plane) bearer setup in 4G. Instead the UE is send to 3G or 2G to establish the voice bearer there. two options for iRAT mobility: blind redirection = UE must find best 3G or 2G cell by itself. And Handover: the radio resources for the UE are already allocated in target RAT before the UE is told to move. The handover is way faster. With blind redirection you need to wait up to 10 sec. until you hear the ring tone after dialing the number
