Really, you mean like the Iranians or Hizbollah that just took over Lebanon?
First, "tend to"
Second, it's tough to make that kind of comparison without getting into "what if" scenarios. Just how dangerous and violent would the Iranian mullahs and Hezbollah be as underground organizations? And how different would whatever other government in those countries be if these groups didn't have the influence that they do? (For example, I'd argue that current Iranian government is, in effect, a pretty secular oligarchy with islamist window-dressing-- the Revolutionary Guard actually runs the country, and gets the official stamp of approval from the clerics whenever they need it. Remove the islamist elements, and not much changes.)
My core point is that I think the risks of Egypt going hardcore-islamist are distant, easily overwhelmed by the overall benefits of democratic reform.
Iranian government is the most dangerous in the world (well China is a lot more powerful so I suppose that's arguable) and it is because of their Islamist ideology. It is far from secular. Also there is no such thing as an islamist democracy, islamism is anti-democratic at its core. After reading various sources the last few days, there's virtually no doubt the MB will a major part of any coalition government. The only question that remains is how much and for how long the army and the other factions will be willing and able to restrain them.