I have no experience with Kindle Scout because I don't like the submission and publication agreement. There's a 45 day exclusivity period where you can't sell the work or submit it to other publishers (and if you've been around the query and agent forums much, you might've seen the strong recommendation not to give exclusive reading periods to agents and such because it's not in your best interest to literally stop your forward momentum. Then again, if you're pretty much set on self-publishing anyway, that exclusivity may not be as much of a hindrance.) I can't immediately see anything in their agreement that gives you the option to back out or change your mind during those 45 days. Basically if they accept your work during those 45 days, the publishing terms immediately go into effect. So be sure to read it ahead of time and make sure you're okay with it.
A $1,500 advance is not much money. I don't personally know that Amazon's advertising plus that small advance is enough to make me give up a significant chunk of royalties. With Kindle Scount you'll get "royalties on net revenues at a rate of 50% for eBooks, 25% for audio editions and 20% for translations." When you publish directly on Amazon, you get 70% as long as your price is $2.99 or more. You're also locked into the contract for five years (you can get your rights back after two if the previous 12 months' royalties have been less than $500 total).
You only have 30 days to make "reasonable revisions" after your work is accepted. And if you don't make any in that time, they can move forward with publishing your work as you originally uploaded, regardless of whether you think it's 100% ready. Basically you better have already paid for a good editor prior to even submitting to Kindle Scout, because Amazon will not be going through edits with you like actual publishing houses will.
To me, Kindle Scout is you still doing most of the same work you'd have to do when self-publishing, but then handing the reins over to Amazon at the last minute for them to take a chunk of your royalties in exchange for you getting their stamp of legitimacy.