Not really true. Both Sunnis and Shia have extremist wings, and it would be very difficult to determine which is "more extreme."
Conveniently, these radical wings hate each other, if possible, even more than they hate us.
Neither the Shiites nor the Sunnis are friends of the “infidel,” shall we say, and both have caused trouble for us (although there are more Sunnis, so therefore, there’s more trouble).
They do hate each other, but I think it’s been a mistake for us to rely on this and to hope that by supporting one side or the other in their battles, we’ll be able to keep them off balance and fighting with each other. That was basically the reason we didn’t take out al-Sadr in Baghdad, even when he was leading Shiite attacks on us: the theory was that he would keep the Sunnis too busy fighting him to go after us. I thought it was a foolish strategy at the time and I still think it is delusional to hope to control the situation by supporting one or another of these groups and hoping that they all kill each other off in the end.
They won’t, and actually they seem rather strengthened by the conflict; it has certainly benefitted ISIS to be seen not only as succeeding against the infidel, but as having success in “purifying” Islam by attacking less populous sects. Something like 80-90% of the world’s Muslims are Sunnis, and the Caliphate is their project and has been since the beginning.