I think Ingram Spark just became my go-to for print on demand. I have always liked the quality of books printed by Ingram a bit better. Up until now because Amazon KDP print books had free setup and an easy approach to getting printed proof copies made going with Amazon KDP first seem smoother. All good reasons to go with Amazon KDP first or perhaps as the only printer.
Free Setup at Ingram Spark - Their new pricing now includes free setup and free updating, at least in the first weeks after a book goes live. While they have always had some 'free setup' offers it looks like this is a permanent change.
Why Ingram Spark over Amazon KDP? - Ingram has a more thorough file evaluation prior to printing and provides good feedback where there are problems. They seem to have tighter spines and over all a bit higher quality. Ingram claims to have the one of the publishing industry’s largest global book distribution networks including Independent bookstores, Online stores, Chain stores, Ebook retailers, Libraries and Universities.