Answer from David Kudler, publisher at Stillpoint Digital Press:
Lightning Source (LSI) and IngramSpark (IS) are both portals offered by Ingram Content Group (the world's largest book distributor) to access their print-on-demand (POD) and distribution services. They are very similar, but have a few minor differences.
LSI is older — it was a standalone company founded in 1996 and acquired by Ingram about fifteen years ago. It was intended to offer publishers of all sizes POD services — especially useful for low-selling backlist titles, but also for popular titles that outsold their print runs.
Ingram started IS in 2013 specifically as a service to the burgeoning micro- and self-publishing community — allowing them access to the same POD and worldwide distribution as major publishers, and allowing them to release their books in print without the high-risk investment of printing thousands of copies of their books (and then having to store them and ship them).
They both use the same worldwide network of printing plants. They both offer the same access to Ingram's distribution network. The print quality is the same.
There are differences between the two portals, reflecting their different origins.
LSI allows the publisher to offer discounts from the industry-standard 55% (which ends up being a 40% discount to retailers) down to 20% (8% retail discount). IS has a set 55% discount. (Some publishers like to be able to lower the discount in order to drop the price for certain books. It generally means you won't get the book into any brick-and-mortar stores or libraries, but they may not be your target anyway.)
IS offers ebook distribution (they take a higher than standard 20% cut); LSI is print-only.
LSI charges an annual $12 "catalog" fee for each edition, and has slightly higher fees for revising your books (I avoid the setup fees as a member of IBPA).
LSI offers printing discounts on titles that sell well (BoLT).
As of this month, ScribeCount (the online sales tracking service) imports sales from IS, but you have to hand-enter them for LSI. Other than that, they're essentially the same.
I use LSI, but only because I started before IS existed. I have played with "short" discounts, but mostly stick to the standard 55%. The ScribeCount situation has me seriously considering switching, but… if it ain't broke, don't fix it.
For what it's worth, I strongly recommend that my clients use IS rather than LSI to distribute to the book trade — it's easier to set up, has slightly lower costs, and most of the high-end features of LSI aren't going to come into play. Then we use Amazon's KDP Print to distribute to Jeff Bezos's domain.
Ebooks, of course, are another issue altogether.
Thanks for permission to reprint this useful post to David Kudler.