If no one has rebuked you or called you childish names for criticising Christianity, may I suggest you're not doing it properly..? ;-)
On a general level, there is a difference between Christianity and Islam in that most of us grew up with some measure of Christianity in our lives; it's something we're pretty well up on when it comes to criticising it. Islam doesn't fall into this category for most people. I'd wager that 9/11 significantly increased people's knowledge and awareness of what Islam is about, but also added more disinformation and fear-mongering too. Add into that the fact that for some their anti-Islam sentiments are actually just a cloak for the fact that they're your bog-standard racist. If followers are defensive, there may be some reasons for being so. Islam is also a younger religion, and perhaps like a petulant teenager, its followers are more inclined to fits of pique. Give them another five hundred years and if they're still around they'll probably respond to criticism by having a bring and buy sale.
None of that, of course, affects one iota the fact that Islam is just as silly and made up as Christianity.