I think A Case for Faith is the better of the two books. Both books are very one-sided as Strobel only interviews Christian scholars who support his viewpoint and doesn't include any detractors.
The Case for Christ is largely about historical evidence, and while I agree with most of the conclusions, I don't think Strobel is vigorous enough in his presentation. If you are using this book for witnessing, I think atheists will be able to poke holes in much of it.
The Case for Faith, on the other hand, is more philosphical. Strobel presents good answers to tought questions Christians face such as how a loving God can send people to Hell or how God is not responsible for evil.
They're both good books worth reading. He also just came out with a new book called The Case for a Creator. I'm probably going to pick this up soon