You could look at it like preservation is the weaker version of lifekeeping, only keeping the Corpus from rotting, but still requiring a lifegiving to bring the Anima back, while lifekeeping is the better version that you only need to fix the body to bring the target back to life. . .if you look at the list, the preservations come at lower levels with longer durations. . .if you need to drag dead guy a week back to civilization to be healed, you might need to keep casting lifekeeping over and over and over many more times than the longer lasting preservation spell of the same or lower level.
That's generally been how I play it myself, though I have had GMs who felt the "Preservation" affected the Corpus, while the "Lifekeeping" affected the Anima, and neither "crossed over". In that case you'd need one to keep the soul present, and the other to keep the body from rotting.
I personally would not go that route, unless I was specifically trying to make life restoring magic even more difficult in an effort to reduce it's use in play. (Some game worlds benefit from lifegiving magic being either extraordinarily difficult / rare, or from not allowing the list for PCs at all).