You get QT-X no matter what.
The option is for QT-7 player. But, if you already have QT-7 Pro, it will detect that and automatically install/move the player [to the Utilities folder].
From what I read in Ars Technica review, QT-7 is always installed at the system level. In other words, if there's a codec needed from 7 that isn't in X (like Sorenson), QT-X will automatically create a new process to handle that (a trick to get around the 32-bit/64-bit mutual exclusion problem). It's only the player itself that's optional. (And, as mentioned in a previous post, allows apps like FCE to continue to operate normally.)