it still uses IngramSpark's distribution
Yes, indeed: with Draft2Digital you seem to get a far more user-friendly service all round, as well as the benefits of Ingram distribution, which leads to automatic listings in all sorts of established stores' online catalogues.
I had a query about the universal book link which I sent into D2D on a Sunday, not expecting a reply until the week begun...and got a personal response within a couple of hours. Impressive.
If you want, you can swerve KDP and publish via Draft2Digital and they'll make sure your ebook is available via Kindle and the print version from Amazon. Personally, I like having the control on territory pricing etc. that publishing via KDP offers, so my preference is to use both platforms.
I purchased my own ISBN numbers so I know I'm solid on the ownership of my titles and can have a free choice of platforms without worrying about possible restrictions further down the line.
Also, if you choose to publish with Draft2Digital, you forego the ability to enrol your title into
Kindle Select, which adds it into Kindle Unlimited and can (potentially) be a useful marketing tool, reaching many more readers.
In order to be in Kindle Select you have to put your title exclusively with KDP for a 90-day minimum. If you want to do that, you could first publish via KDP, do Kindle select for a few months, then take your title off Kindle Select and go ahead with Draft2Digital in order to "go wide". You can, if you wish, delist your title from D2D, so it doesn't preclude a later enrolment into Kindle Select... but I suspect that gets fiddly.
Draft2Digital offer to enrol your title into Kobo Plus, which is Rakuten's equivalent (more or less) to Kindle Unlimited. You are also offered the opportunity to include your title in an annual SmashWords promotion, so there is that!
Hah, I reckon D2D owe me a commission after all that!!!