I'm not sure what "library quality" means, but it's my understanding that all of these (and blurb and iPhoto) are using HP Indigo presses. Assuming they are operating their machines within spec, the real differences will come down to paper and binding options, customer support, and how much flexibility they offer in their workflow.
Not all the process (printing) are necessarily Indigo. There are differing models of Indigo. More importantly, there are a lot of ways to setup and run an Indigo, like most presses, you can 'conform
' to a standard if one even exists or attempt to exceed the spec. Those that attempt to exceed the spec have varying degrees of success doing so. Some folks want to use as little 'ink' (Toner in this case) as possible because that's expensive. Some don't mind, they strive for the best possible quality, higher TAC. Indigo's have a lot of expendable items that can (should) be replaced depending on how well the press operators are examining the output. If
they are examining the output at all. IOW, there's a huge number of variables that can make a difference in the quality of even the same Indigo press at shop A vs. shop B.