The difference is the accuracy. A matched pair of C3 anglular contact bearings probably cost about 10 times what a C7 set cost. I'm sure you could use a C7 fixed end wih your C3 ballscrew, but then you effectively make your C3 ballscrew a C7 ballscrew,,,
The bearings can also be arranged face-to face, or back to back. I believe the face to face method provides more preload ability and accuracy, but don't quote me on that.
BTW, you're not preloading the ballscrew, you're preloading the bearings, which creates the "fixed" end.