I only have experience using Amazon's KDP, and a bit of Ingram Spark, plus printing myself and selling through Amazon marketplace and my own website.
All depends on what you need/want really. IS gives you access to distribution channels, so people can order via non-Amazon shops/websites, whereas KDP is of course Amazon-exclusive. Both will take all your money (
) though IS is slightly worse. It's worthwhile using both though, so you can sell via Amazon as well as other shops.
Dealing with Amazon is exactly how you might expect. It all works
relatively well, but if things go wrong then you may as well go and scream at the sky. It's not like the old days when you deal with real people.
Both of the above relate to print on demand. Printing yourself is very different and does open the possibility of actually making a profit (!)... but you'll need to know that you can sell several thousand copies in advance somehow, plus be prepared to do all the work/posting etc. Doing everything yourself (ie. selling through your own website, fulfilment, taking the orders, posting) is an awful lot of work and a steep learning curve, but you do get to keep all the profits. A halfway house is selling via Marketplace, which obviously eats into any profit but does avoid the need to have your own online shop, plus you get some exposure via Amazon. Again, I've done both together for some books as a way of hedging my bets – maximising profit for direct sales while trying to drive more through Amazon.
If going the 'full' route of printing yourself, then you can deal with distributors to actually get the books available for Amazon physical shops etc, so you're essentially mirroring the entire trad publishing model. But, again, there are costs involved and you need to be in a situation where you know you have sales in the thousands waiting for you to make it worthwhile.
The only other routes I can think of are things like vanity publishers, which are the worst of all worlds and to be avoided like the plague. I don't really understand the point of those, but I guess they work for people who just want to pay someone to do all the work I guess.
In all cases, it's important to understand that nobody apart from you will be actively selling or marketing the books! I'm sure you understand that part of it, but it's amazing how many people assume that if you publish something that somehow people will automatically buy it! Sales will be approximately zero unless you find people and persuade them to buy it, via whatever marketing means you have planned.
Happy to answer any more specific questions you have.
