Phase 1: Math 54 vs CS 61B

I’m trying to take Math 54 and CS 61B next semester, and I’m having trouble deciding which one to sign up during Phase 1.
I have heard that CS 61B generally accepts everyone off the waitlist, and that I should add it during Phase 2. However, if I want a decent discussion/lab time, should I Phase 1 it instead? Or will I generally be able to switch discussion/lab times later in the semester (allowing me to wait for Phase 2)?