There are pros and cons to both genres. CC gives us the possibilites to make what happened on the show believable and even add in scenes between particular episodes and such while AU gives you much more creative license with plot and characterisation. Though writing-wise, I definitely see more conventionial stories involving Alex and Isabel than alternative universe.
For myself, I'll admit that I prefer to read and write conventional stories. I find that my creative license only goes so far, so writing alternative universe takes a bit more effort even if I love the challenge. When it comes to Alex and Isabel, there are many avenues that have not been explored with conventional stories, especially centric fictions. As you may have noticed with my writing, I tend to write centric fictions post- "place particular episode here." Analyzing characters and displaying different aspects of them while keeping them in-character is my specialty, or so I like to think.
Now when it comes to reading, though I have said I prefer to read conventional, I still like to read AUs. My issue with some AUs is that, like any story, it must have a plausible premise (that I can picture) and be in-character (unless there's a perfectly good reason why they're not). I'm also a stickler for proper punctuation and grammar, (to the best of the author's abilities, of course), but that's with all genres.
So the verdict: I prefer conventional, but do venture into AU when the desire arises.
