You were arguing about their motivation, not their frequency. And if it's frequency, the Christians win in the US.
Let's see - seven people have died at the hands of abortion clinic bombers. How does that compare to the events of 9/11/01?
Why limit it to just the US? Let's compare all abortion clinic bombings that had fatalities to all suicide bombings that had fatalities for the planet over the last 20 years or so.