I think the only way for you to decide is just to sit down and listen to both WTCI and WTCII from beginning to end and pick the one with the most P&F combinations you want to learn. There are great pieces in both sets. The other question is whether you really have to limit yourself to one or the other volume. There are many editions available on IMSLP, so if you like some in volume 1 and some in volume 2 it's easy enough to get the ones you want.
I am partial to book 1. I think it has more varied technical challenges and overall better / more interesting pieces in it. It definitely has the most of the famous ones! I would go with Book 1 if I had to pick.
Yep, I've decided to go with that one since it'll be more beneficial to me and you're right, the famous one are definitely all in there oddly enough.