Ulysses and Scrivener both have forms of versioning control. Ulysses does it automatically when you manually save and Scrivener achieves that effect when you "take a snapshot." I like iBooks Author because it reminds me of the old Pages, but I don't use it.
Trials are really a great thing. I tried the Ulysses trial again on a new system, imported some of my notes from scrivener and really REALLY wanted to like it. On it's face, it seems like Ulysses would be perfect for me, but I just can't "get with it." I don't like how bolding something makes it pink, and italics make a word blue. You can customize the theme but that's overly complex. There are no inter-page links and although you can assign keywords, if you right-click in the tag area it clears them all!
Had I paid $45 for Ulysses I would have been bitterly disappointed.
I really want to like it, but I just can't seem to get with its flow. Also, it kind of goes against what Markdown stands for. Markdown was supposed to be a universal format so you could take your files and use them with any markdown editor and never lose stride, but Ulysses keeps all your files hidden in the library as unreadable ".ulysses" files so to move to another app you'd have to export each section or manually copy and past over to a new app. That is WAY too much work. Scrivener is no better about that, but it doesn't feature Markdown as a key feature. Ulysses is a markdown app, so it should abide by Markdown's app-agnostic nature. Of course, this is all my opinion.
I really can't wait for Scrivener for iPad. They claim it's done and in testing with a release scheduled for "late summer" but that will probably be around October (I'm guessing). Ulysses has the iOS side down, but they've been having issues with iCloud Sync. Scrivener is only utilizing Dropbox for the time being because of iCloud's finicky nature and I wish Ulysses offered that too. Ulysses wants to store your notes in iCloud, which I don't trust, but they do have a new automatic backup system. Also, and this may be of importance to you, there were some reviews saying that Ulysses had stability problems with large walls of text (45,000 words I think). If you're going to write a long book, it may not be the best choice.
I'm excited for Apple's new Notes app. I think that will be my go-to place for my thoughts when I'm on the go and when I get home or sit back with my iPad, I can take those notes and put them into Scrivener (split screen on iPad finally
). I doubt you'll see a Scrivener iPhone app, whereas Ulysses says one is coming.
I am not trying to discourage you from using Ulysses. Try it and you may love it, but I am trying it for the first time too and just wanted to share my thoughts. Also, I had some funky bugs with the trial version creating duplicate notes even for local files? :/ There have been reviews on the Mac App Store about crashing problems. I can say that in the year I've used Scrivener even for really big projects (several hundred MB thesis with 80,000 words, 30 PDFS, images, etc.) I never had a single bug present itself or a single crash. The only think that takes a minute (usually with the beach ball) is when you compile a large document for export, but that's not the app freezing, it's just...well...compiling. I'm sure there are bugs and crashes, but I've never had a single hitch with it in my experience. It's one of the most stable apps i've ever used.
Honestly, the OS X app doesn't tempt me at all. The iPad app still tempts me because it is one of the few FULL featured writing apps for iPad. But without it's companion OS X app, what's the point? I'll wait, with great hope, for Scrivener.