Since it sounds like you're keeping a personal BoS not attached to a specific tradition that would rule-bind you to do things a certain way, you can do what you want. What the differences would be are up to you and your own personal metaphysical worldview.

To me, a BoS is not at all the same as a grimoire (spellbook) so I would never consider having one substitute for the other. To me, the big consideration is usually organization and ease-of-use. From an organizational standpoint, it might be reasonable to keep spells in another volume. I keep most content remove/add-friendly, so it would be simple to swap around pages from one volume to another at a later date.