Wow. What a fun nightmare!
I rode the 5.2 Performance with the compact Ultegra SL cranks. It was an awesome bike, and if I weren't such a titanium snob (said with tongue planted firmly in cheek), I might actually own one now. I don't have a link to the thread now, but if you search the Bike forum here back to October, '07 you can find my review of it.
The Ultegra SL group is oh-so-close to DuraAce in performance, so the price savings is really attractive there.
I've heard, but cannot say for myself, that the difference b/t the red and black carbon is night and day. If the 5.2 has a wonderful ride - responsive, tight, fast, yet very comfortable - I can only imagine what the 6 series is like!
I agree with Liza....go out and ride them and make your decision that way.
What are you looking for: Speed? Comfort?