I went through Wiley, which is a very traditional, large publisher (my editor is the same as Mitnick's and Schneier's). You seriously limit the royalty rate you're going to get this way, but you're going to extend your reach and marketing ability. So it's a balance, and you need to figure out whether reach or per-book royalties are more important. If you have a small audience and you know you can reach each and every reader yourself (maybe on Twitter or HN or Reddit), then self-publishing may be the way to go.
I chose Wiley, because the audience for my book is broader than most data science books (since it's spreadsheet based), and I wanted to get it in front of your average BI analyst perusing at Barnes and Noble.
The big publisher also did a great job with layout, the book feels great with its matte finish, I had two paid tech editors, a regular editor, a project editor, and a bevy of proofreaders. Big help, that.
But don't be mistaken...the onus is still on you to market the book as well. And it's going to take a lot of marketing to make this a viable career (or even a viable supplement to your income). It's an incredibly taxing way to make a living I'd imagine, and I'm super happy to have a great day job.
Rather, my reason for publishing was that I saw a need for the book, and I knew I could write it. And it took a full year from starting to get it out. So it took a year of hard work, and it'd be a miracle if I cleared twenty grand on it. Not the most lucrative endeavor, but extremely personally satisfying.
To answer some of your other questions, the publisher released in both paper and ebook. Pictures in the book were a real pain in the ass. I spent forever screencapping and re-screencapping spreadsheets. Ugh...