Someone on the...chicken? forum?
/me checks Amy's signature. No chickens.
Anyway... I haven't read either of those, so I can't give you specific ideas. As others said, usually it is best to go with what you enjoyed. You might be able to do well with a "why I didn't like this book" type of thing, but that is dependent on knowing how your professor will receive it. Also, it depends on why you enjoyed one more than the other. If you just didn't like the writing style, you probably won't get far with that. But if you didn't like the ending, and can come up with good reasons (characters didn't act like themselves, etc), that could be fun to write and come across very well.
While this shouldn't be true, the best way to get a good grade is to know what your professor likes to see. You should be able to have a good idea of this from sitting in class, even if he isn't helpful in talking to you one on one. Does he talk mostly about symbolism? Characters? Plot? Comparing & contrasting 2 different works? Does he point out things that don't quite "work"? Or is he more in the "these are classics, they are perfect" mode?
Don't be afraid to start writing your paper when you have no idea what you are going to say. Some professors will be very accepting of a well thought out, well written paper that has a conclusion which is basically "I have no idea". Even if you feel your professor isn't one of those, the writing process itself will often clarify things for you or give you ideas about an entirely different subject which may make a better paper.