While there certainly are other aspects to be taken into account, one main consideration when choosing a repertoire based theory book, is the quality of the recommended lines.
In my and GM Leif E. Johannesen's "The Ruy Lopez: A Guide for Black" we primarily recommend the Zaitsev variation, starting 9...Bb7 10.d4 Re8 11.Nbd2 Bf8.
In "A Spanish Repertoire for Black" on the other hand, GM Mihail Marin recommends the Chigorin variation, starting 9...Na5 10.Bc2 c5 11.d4 Qc7.
So, what is best - Chigorin or Zaitsev?
Obviously there is no definite answer to that question. Both lines have been played at absolute top-level and have withstood detailed computer assisted analysis. But there nevertheless are some differences that may help you decide:
- Zaitsev development is quicker and more natural.
- Black's knight on a5 tends to create problems in the Chigorin.
- The Chigorin is the oldest line, and has developed a correspondingly larger body of theory.
- The Zaitsev tends to lead to sharper lines where both players must follow a narrower theoretical path.
- In the Zaitsev Black must worry about weaker players going for a repetition of moves.
- In the Chigorin Black tends to be a little passive for quite a long time.
- In the Zaitsev mainlines Black's kingside tends to come under fire.
- Experience pays well in the Chigorin and your results are likely to improve over time.
- Strategic ideas tend to be clearer and easier to understand in the Zaitsev.
- Chigorin strategy tends to be extremely slow and often quite subtle.
- In the Zaitsev mainline the centre frequently becomes quite open.
- In the Chigorin White can close the centre permanently with d5.
In the meantime it's nice to know that even if you should happen to choose the wrong line (it could even be that the Breyer variation is the line for you!), there will be very little work wasted. Your study of one Closed Ruy Lopez line is almost guaranteed to deepen your understanding of any other line within that complex. And whenever you decide to do a switch you will save a lot of time.