All I can say about KDP Select is that, it's toxic. As long as you aren't very successful with KU page reads, you're fine, but promote to audiences that target KU books? Amazon will accuse you of gaming the system, defraud you of your royalties, and threaten you with account termination. I am told there is a big class action lawsuit against Amazon either pending or in progress over the matter.
I have several books listed with both D2D and Smashwords (and yes, Amazon) and lately I'm doing the best with D2D. One word of caution - both D2D and Smashwords has their own quirks that prevent you from porting over your Kindle books without modification. Both accept and prefer Word documents for upload, but with D2D, you have to convert double carriage returns to triple carriage returns or you will not have section breaks. Also Kindle compliant covers do not look correct on some of the retailer previewers and the aspect ratio must be modified. With Smashwords, covers, text, etc... all look great but their uploader cannot parse a Word TOC, and you have to create one using a funky, protracted bookmark procedure.
D2D has a slicker interface, but they do apply some autoformatting that can cause some issues. Smashwords I think is a more mature, stable platform but that said I don't sell squat through Smashwords.
As much as I have a hatred of Amazon, I will say it offers the best quality product, and it's fairly easy and straightforward to offer print versions through KDP Print. They also recently added a feature so you can order author copies at the cost of printing and delivery.