They're a much bigger threat to Europe, partly because we're closer geographically, but more importantly because they already have a European invasion army in place.
In the last decades, millions of Muslims have immigrated to Europe as "asylum seekers". Now that they have their European passports and citizenship and have collected enough money off welfare, state-sponsored mosques etc, they're going back to their home countries to join ISIS. Right now they're taking the fight there, but in the long term they have their sights set on Europe.
While not all of them were radical islamists when they arrived in Europe, they have been converted and radicalized during their time here. Islamist leaders in Muslim dominated districts of European cities have declared Jihad against the host country. By keeping the districts off-limits to police and security workers (by torching cars and throwing rocks, home-made bombs etc. at police who try to enter), they've been able to indoctrinate their followers without interference. In many such muslim-dominated districts, Shariah law is in place, and popular support for ISIS is over 80%, higher than in Gaza.