It depends on what your distribution strategy is. If you aren't trying to get your book into Barnes and Noble I would go with the KDP Select option. I use it exclusively.
The big downside of choosing KDP's "Select" option for a particular book is that you can't sell that title anywhere else and that includes your own website! If you want wider distribution, then pass on the "Select" option, but definitely get it up on KDP.
If you are just starting out, and don't know how your book will sell or want to test the waters on Amazon, choosing "Select" knocks out other books that are't in the "Select" program which gets your book in front of a wider audience on Amazon. It gives it slightly better odds of getting seen and hopefully purchased by more people. "Select" worked really well for new authors when it was first introduced but too many authors are using it now so it has lost much of its edge.
Another downside of "Select" is that you only get the advantage of the "Select" royalty split if someone buys your book. If no one buys it on Amazon during the 3-month exclusivity cycle, you get nothing. The $13 million pot is split among the authors who made sales, while you lost out on exposure time at other sales channels where you might have sold copies, like your own website, itunes, etc.
I personally only use the "KDP Select" option for certain titles I know sell better on Amazon, and only during certain months. I have one title that hasn't sold a single copy on Amazon since 2011 but does very well on iTunes and a few other places, so while it's listed on Amazon (because you never know some day I may sell one there), I never enroll it in "Select."
The beautiful part of the "Select" option is it's only a 3-month commitment so you can enable or disable it whenever you feel like it. You aren't locked into it forever if it doesn't work for your book.
Ultimately, you have to weigh the pay off for yourself as every book will have unique sales properties and audiences.
Hope this helps. Good luck!
I didn't know you could sell books on iTunes. I'll have to look into that.