New breed - Daedulus
Daedulus is iPad only, and has a unique UI. Instead of the file and folder metaphor, or the two column approach, Daedulus uses "stacks." A stack is like a folder. Sheets can be created within it - and can be flicked around within the stack, or across them in one of the best uses of the touch screen I've seen. I'm writing these reviews in Daedulus, and it's a lot of fun. With the ability to shuffle the sheets, and search across them, this is far better than Plaintext.
While the basic version is free, to get Dropbox syncing and exporting, you need to shell out some cash. I bought the bundle for $3.99.
Daedulus syncs up with Ulysses III through iCloud, I understand, but I haven't tested that. Although I have a Mac (my son's abandoned laptop), it's not really my basic platform.
It is Dropbox enabled - the files are syncing to a Daedalus folder within Dropbox.