"There are significant and little appreciated differences in the trajectory of Sunni extremist terrorism and that of Shi'a extremism. The differences exist across six key areas that impact American policy considerations [...]. First and foremost, Sunni radicals and Shi'a extremists differ in the overall approach and main objectives for their use of terror. The former tend to operate in a continuous, mid-to-high intensity manner, seeing war against infidels and apostates as a perennial condition featuring overlapping waves. Outside of an ongoing and seemingly open-ended campaign against Israel, terrorist attacks by Shi'a groups have by and large featured discrete terror campaigns tethered to state and organizational objectives. Second, Sunni terrorists and Shi'a extremists manifest different patterns for recruiting terrorist operatives and developing terrorist missions. Shi'a terrorists, unlike their Sunni counterparts, enjoy direct state support and for that reason are far more likely to originate from Iranian embassies, consulates and state-run businesses. Third, despite holding a minority viewpoint within the wider Sunni Islamic community, Sunni extremists, especially Salafi-Jihadis, rely more extensively on the support of their coreligionist expatriate communities in facilitating terrorist activities. Fourth, while employing similar tactics and methods, Shi'a terrorist groups have shown a much greater propensity to kidnap innocents to barter, while Sunni extremists more frequently abduct to kill. Fifth, Shi'a terror groups exhibit a much higher incidence of targeted assassinations for specific political gain, rather than the high-casualty killings featured in Sunni terrorism, and particularly of the Salafi-Jihadist variant. Finally, each sect's extremists manage publicity and propaganda differently."
Combating Terrorism Center at West Point: http://www.ctc.usma.edu/