I think to say something like that you should take into account the amount of prevented attacks and the overall use of police resources to prevent terrorism.
I mean, it could be so that the other group has less succesfull attacks than the other but the police has to use their resources much more on the former group than the latter because there are more planning of attacks and more attempted attacks.
I have no data to prove anything about that in any direction and I'm not sure if data for that even exists. But at least in Finland the situation is this:
"The most significant terrorist threat in Finland is still posed by individual actors or small groups motivated by radical Islamist propaganda or terrorist organisations encouraging them."
"The Finnish Security Intelligence Service (Supo) has around 350 counterterrorism target individuals."
http://www.supo.fi/counterterrorism/terrorism_threat_assessment
That was written
before the first islamic terrorist attack happened in Finland last August.
And note that the link is the official website of The Finnish Security Intelligence Service. It's not any sort of right wing propaganda.
It would be interesting to know how much the US Intelligence is preventing or investigating Islamic terror plots versus preventing or investigating Christian terror plots. How many "counterterrorism target individuals" they have who are Christians versus who are Muslims. Or of any other religions or groups. Just because the attacks might not happen that often doesn't yet mean there isn't danger of that. It could very well be that there are more people who are planning attacks but the US Intelligence are doing their job well enough to stop most of the people before anything happens.