All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
Both books which you mentioned in the question are good and beneficial and you may benefit from both of them. However, each book has its own characteristics. The book 'Fiqh As-Sunnah' edited by Sayid Saabiq is more comprehensive than the book 'Sifat Salat An-Nabi' as it comprehends all or most of the Fiqh chapters, whereby the book 'Sifat Salat An-Nabi' is about the prayer only. Nonetheless, the characteristic of the latter is that it is more concise in clarifying the method of the prayer of the Prophet .
Therefore, we advise you to benefit from both books as each one includes matters that the other does not. The book 'Sifat Salat An-Nabi' is more authentic and better in clarifying the method of the prayer of the Prophet and its author is more knowledgeable in Sunnah.