is that "Prep to Improvise" seems to me to imply that you're doing your prep with the idea it will be the foundation of your improvisation whereas "Prep what you don't feel comfortable improvising at the table" feels to carry more the idea that what you improvise won't be, might not even be related to, what you prep.
For example, in prepping for the session I ran last Friday (session one of a new campaign, but that might not matter all that much--though I do prep campaign starts more than just about anything else) I worked out like twenty names, complete with Lineages (ToV-speak for ancestries or species) but no other personalities. If I'd needed an NPC I'd have grabbed that name and Lineage and just dropped it in as the kind of NPC I needed, if that makes sense. Those names were just names, and it turned out I didn't use any of them, and I improvised a lot of other stuff not related to those NPC names at all.